I've done this many times when swapping from a Fox 35 to an OS 25.
Wash the front of the plane with something that takes off oil, like denatured alcohol or even actone.
Redrill the old mounting holes and plug with dowels & epoxy.
Rough up the inside of the engine mounts with a Dremel and epoxy on some plywood shims. Grind the the shims so the engine fits tight. Redrill the holes to the new engine.
Fuel proof with epoxy finishing resin.
This works fine. Not quite as good as doing it right the first time, but still a lot better than going to the grave with a Fox 35 Stunt.