I have a 1968 W200 with a stock 318 and 4 speed. The original starter is mounted with a three bolt flange (OEM).
I bought a mini lightweight starter to install but of course it does not match up. My question is, are there any mini lightweight options or any adapter plates to accommodate the (new OEM) two bolt version?

Is this what your starter looks like ???
It's what Rock Auto shows as being correct for your combination.

Yes. That is the OEM.
Those are easy to find (Classic Industries, O’Rielly,..).

Well the odds are that any of the two bolt mini-starters starters will never work with your combination.

It's probably going to take a bellhousing and probably flywheel change to use any of the normal two bolt mini-starters.

The transmission that you have will also affect what bellhousing that you can use.

Can’t he use a newer starter from a 94 ram, for example? It is lighter, smaller, and better than the old ones?

No way will the mini fit Look at the difference The OE is a 3 bolt and huge Looks like the old style they used to run years ago.

Agreed.
Hence my question…

Lets start from scratch, are you having problems with that style starter?

Is it cranking really slow or some other issue?

If it is, cranking slow, simply replacing it with a new or fresh rebuilt one may cure the problem.

Later model starters simply crank faster that the old gear reduction did.

What you have isn't the typical gear reduction Mopar starter.

It's more like a Chevy in some respects with the solenoid on top of the starter.
They used that style starter on 66 & 67 Hemi's with a 4 speed and some slant sixes back then.