New engines for the Ercoupe

Under the rules of the STC process, once the FAA grants you the STC number to begin the process of developing the STC, the FAA will temporarily re-classify the certified aircraft as Experimental. The Experimental status will revert back to certified once the STC development work has finished.

Actually you can. You apply to the FAA for an Experimental Exhibition, not Experimental, Amateur Built. If you are approved, you can do pretty much what you want. First time I ran into this, a gentleman was modifying a certified aircraft to generate an STC. I did not hear if he succeeded or not. I do know the FAA is very sensitive about this so it would probably be a bit tricky to get approved.

You cannot make a Coupe experimental. Bad advice. To register a plane experiment, you have to prove you constructed it. No certified plane can be downgraded to experimental.

Two ways, one easy, one harder.
Easy way: Make the airplane EXPERIMENTAL and do whatever you want.
Not easy, find a mechanic/shop willing to do a 337 for a 912 motor. I think it would require a field approval and good luck with that. I have not heard of anyone doing an STC for a 912 but you might see if one exists. O-200s are available pretty much everywhere and there are LOTS of O-200 Ercoupes.

Agreed, John. Now my question is... How do we do it?

I talked to a fellow at Plant City (KPCM) airport a while back who is working on putting a 912 in his "experimental" coupe. I think, in light of the fact that C-75/85's are so hard to find, it would be a great idea to put a 912 in.
John