Looking for something of an (externally) adjustable engine thermostat so operating temp can be adjusted.
Preferrably something that can be adjusted through the housing with an allen wrench.

After Googling I found some modern electrically controlled thermostats, but nothing mechanically.
Anything out there?

What do you want to achieve? The thermostat is only there to get the engine to operating temp quick and make sure the heater works in cold weather.

I run a standard 180 stat, and an electric adjustable cooling fan control. Engine warms up quickly to 180 and then I can adjust the fan to come on where ever I want. I also have a switch mounted under the dash so I can run the fan continuously if I want. Gives me full control to run the engine temp where ever I want. Of course you have to have enough rad ( and fan )to do this. If I want to run at 190, I set the controller so the fan comes on at 190. Temp usually oscillates about 10 degrees as the fan kicks on and off.

first I'd get a top notch gauge so that I know what my temp REALLY is. On your Q if I was to do that I'd look into an adjustable valve to plumb into the top rad hose (cable operated?) to adj the flow & regulate the temp that way

I'm running a 160 degree thermostat in the engine but during these colder outside temps, engine temps hover between 150-160, and because of that the car's inside cabin heat is lacking, so I would like to be able to simply turn up the engine temp to 180 during these times.
The engine runs on propane which likes the lower 160 operating temperature.

An electrical fan might do the trick, although the stock (wornout/free wheeling) thermo-clutch fan and fanshroud have no problems keeping the engine cool, so installing more electrical stuff in the car for this seems a bit overkill.

Ok I see what you want. Just put cardboard over 1/3 of the radiator and watch the gauge. You can cover more or less to get what you want for operating temp.

Sounds cheesy but that's why they make winter fronts for diesels.

I usually do this when temps get really low, and currently I've got a shield in front of the transmission cooler.
But I was trying to find a 'slightly' little less ghetto solution to this by looking for a nifty little device, but alas, no-one in the autoparts world didn't see this very useful yet I guess

Although I've seen modern electronic controlled thermostats used by BMW, doing pretty much the same thing. Varying engine temps for different engine loads, effiency and economy.

Less Ghetto looking? Paint the cardboard black LOL

Just a tip, if you cover any of the fan's are do it equally!!! Also cut a hole in the middle so the fan isn't trying to suck forward, hard on bearings in the clutch and water pump to be pulled forward or side load.
I used to not know this until I blocked my 12v cummins radiator off 100% 3 winters of that and I sent the fan through the radiator!

Some times the simple way is the best way.. semis
have been doing it for years... they have a nice
snap on cover.. usually you see it in a V shape
with the top center open.. yeah you can paint it
black to look less getto

Forcing the radiator temperature to rise is a pretty crude (and slow) method of raising the internal water temperature.
It also depends very much on the ambient air temperature, and on vehicle speed: the faster you go, the more of your radiator you have to mask off.

I tend not to do mods that require me to work under the hood while driving.

A few yrs ago I pondered about an adjustable aperture design would be neat, depending on the material used, & how a O'ring/seal would work with that.

Would be something neat to have in one's own study of water speed through a block.

Controlling it electronically is the easiest and most reliable way to do it IMO. All done from the comfort of your pilots seat.

Honestly if it's struggling to get to thermostat temp now, the only way you're going to easily fix it is to reduce airflow.

Well, as so happens, just yesterday we had a recordbreaking warm day on the 25th of January, with daytime temps around 14-16 degrees Celsius (58-60F).

So I seems I don't just yet have to evict the local hobo out of his cardbox house so I can drive around cosy warm...

If the thermostat is closed, what difference does airflow through the radiator make? If the engine isn't up to thermostat opening temp, it shouldn't be circulating coolant through the radiator anyways, so changing airflow through through the radiator shouldn't affect coolant temps.

Airflow through the radiator actually does two separate things. Airflow through the radiator changes the coolant temp in the radiator, lowering (or raising) it depending on ambient air temp. That air then exits the engine compartment, usually out the bottom, and removes the heat trapped in the engine compartment that radiated from the engine, headers, etc. Many times blocking the airflow through the radiator doesn't change the cooling systems heat loss as much as it changes the heat loss through the loss of the "radiant" heated air in the engine compartment.

Many times a chin spoiler will help a car cool down, and people assume it's from additional airflow directed up into the radiator - it's actually from the spoiler lowering the air pressure under the car, effectively sucking the air out the bottom, and the engine compartment air being able to exit easier.

Also, many semi engines use air pressure controlled fan clutches. In cold weather, many times while idling overnight they won't heat to thermostat opening temp, even though the fan isn't turning. The engine at idle doesn't generate enough heat to overcome the heat loss to the outside air, and/or the cabs heater cores.

If you have a hard time getting engine temp up to
the stat temp... put a lean out valve on it.. its
just a air leak to the intake.. most alky cars run
them.. I had one run inside with a small(1/8") valve
with a larger valve close to the intake(1/4" valve)..
that way I could set the larger valve with a set opening
then just turn the valve inside wide open so I didnt have
to just crack the valve open

Most thermostats have a bypass port so a small amount of coolant is circulating all the time, and they typically start to open below their rated temperature.

How did this thread get so long? Your correct answer is right here in the 2nd reply.

The only thing that needs to be added here is the bottom temperature will be the thermostat temperature on the highway. Even with no fan the air through the grille will be enough to thermostat to regulate the temp at it's rating.

The temperature the fan turns off needs to be higher than thermostat temp. If not you will create a situation where the thermostat is closed and your fan is running.

Might be the answer for a street driven only car. If he wants to be able to select, say 130-140* for the track and 180* for the street - not so much . . .