Since the case is round, you can ususally get away with shaving just a bit off the top corners to get a good fit.

I have to admit, I have just shoved the engine in and screwed it down on nylon mounts before, just letting the nylon get out of the way. No problems doing that, but then again, I was doing it on combat planes, and nylon mounts don't last long in combat planes. And it isn't because of vibration or bending stresses either.

If you do just shove it in there, check it once in a while for signs of cracks forming. If you see any cracking, or if it's a black nylon mount, any light streaks showing up, it's time to replace the mount.