Just about any 4cyl street bike engine can fit in a zilla, but they'll ALL need frame work and modifications to bolt in.
Another option is a rotax triple (or suitable 2-stroke snowmobile engine) if you want to keep it 2-stroke.
I've seen numerous examples of street bike engine conversions, and at least two zillas with rotax engines.
One example was built with a 2-stroke Suzuki Gamma 500 engine (Gammazilla), so if you could find one of those engine, GO FOR IT    you'll have a pretty **** quad when you're done!