So I am wondering, if Honda 390 engines are in short supply right now
what other small powered equipment - go carts, tillers, water pumps, etc
have a Honda ( or Kohler) engine that can be transplanted in place of a failed Briggs & Stratton engine?

In vast internet land
is there a forum with knowledgeable small engine users who could suggest transplants?

If the BS de-compression mechanism
is a weak link that can wreck havoc
can it be disconnected
even if it makes pull starting harder?

You need to remove the camshaft. You will see an L shaped lever with a spring. The lever swings out allowing the exhaust valve to stay open longer as the camshaft rotates to make starting easier. Question to your answer is yes. You can remove
the compression release. Remove the camshaft. Then, cut it out from the camshaft. But, you don't need to do this. Chances that it will break are very small. I had one break off years ago on a B&S 15.5 OHV , on a 1996 Murray 42" mower. .
Did not even know it. I notice it was a little harder for the engine to start. But , I ran the engine like this for many years. I did replace it when the engine threw a rod back in 2016. Was not caused by the broken compression release remains. But was caused by an oiling problem. got a new connecting rod and a another camshaft with a good compression release mechanism, rebuilt the engine and has been running fine ever since.

Well on the B&S engine I just replaced the camshaft with a new decompression mechanism the owner brought it to me because he (nor I) could get it to start (consistently) and if we were lucky once it ten attempts. Before I got to it someone sold him a new starter, new starter solenoid, new dual ac/dc alternator and a new battery. I pulled the spark plug out of it and engine spun like crazy. Put the spark plug back in and it would not go past the compression stroke. IMO sure you can remove the decomp but then you deal with the nine out of ten times it will not start. The owner was ready to give up and buy a used Craftsman riding mower just like his broken decomp B&S but a little older for $2200 and told me "Mike it has a new engine".....I said "Dave, why do you think it might have a new engine?" So I fixed his for less that $200 and he's happy mowing again.....sad thing is the new decomp cam showed no signs of any improvement so it's going forward. Personally I wouldn't remove the decomp from the B&S, you might get away removing the decomp on other brands but I know now from experience it's not good to lose it on the B&S. Do a search and you'll find the main issue that comes up when it fails is it's hard to start or won't start at all.

Did you go to the "Small Engine Warehouse" site https://www.smallenginewarehouse.com/?msclkid=b9a0439927b315b12bd822a10a2de687 or better yet call them and ask for tech support. That's what I did for the lady's Troy Bilt '70's tiller and they had a choice of three engines that worked as updates. They can help you decide what might work best for your issue.

Here's the link to over 80 Honda engines they offer, https://www.smallenginewarehouse.com/engines/brand/Honda there has to be something there you can use.

Mike

I did go to small engine warehouse.
I spent more than thirty minutes there,especially in the natural gas fuel section.
I did bookmark it.

I am still interested in “transplants” from water pumps, mini bikes, etc.

My father abused me terribly as a child,
buying me mini bikes, six wheel ATVs, motorcycles.
all of whom broke down,
and I was lured into “addict labor” repairing them with my two brothers.
Addict labor is worse than slave labor,
a slave can work at the lowest effort to avoid the whip,
but addict labor will work with a near frenzy to get back to dangerous thrills.

This “devious dad” even bought us a used 40 hp Wankel engine to transplant in place of a Wisconsin 12 hp.

It got far worse - he bought his sons airplanes after they all soled at age 15.

He drew a line only at helicopters - which he repeated called “10,000 parts all trying their best to kill you.”