Welcome to the forum. You can adapt a cargo rack to the rear triangle and rack-mount the engine with a mounting plate. I wouldn't cut your main frame.
You may be better off trading your rear suspension bike to somebody with a hard tail MTB or steel beach cruiser.
IMHO, the best rear suspension bikes for using frame mounted engines are early GT LTS or K2 Attack. The front triangle of these frames are similar to a hard tail, allowing plenty of room to mount an engine. Soft tails, like the KHS ST series work well also, though they provide only 3/4" of rear travel. Good luck.