Resurrecting an old thread since i’m going through a similar analysis. I want to be able to use either a C85 or O-200 in my LSA. I have both engines and did some measurements. the cases are not identical. doing a rough stack up, the O-200 prop flange will be about about 1/2” further forwarded compared to the C85. the castings are different but most of the difference is in the motor mount ears. the c85 uses the conical mount whereas the O-200 uses the more conventional (and expensive) mount. I will do a more precise measurement soon, but the plan is to position the cowl for the longer O-200 engine, then make spacers as needed between the C85 soft mounts and the engine mount in the plane.
I’ll report back with numbers and pics once i do the detailed stack up. I bought both rubber sets for the two mounts and have to research how much to compress them for a good measurement.