I would sure like to see picyures of what you are seeing.
Not sure who told you that the mounts are different, but the 2 engines themselves are identical.
I used to put all kinds of things in FG models from 28.5 cc Zenoah's and CY models, to 30.5 cc of bothe engine types.
Even put a few TS reed case engines in them too, and they should all fit by using parts from the engine currently installed.
You might have to change clutch housings, and move the engine mounts over to the new engine, but that would be all that needs to be done.
Snap a few pics. or hit some one up to help you get some pics. of what your seeing on here for us.
Hard to help in certain situations without the visual of what is actually different.