Almost every predator build I've seen involves some sort of mounting plate or bracket to seat it on the frame. I'd look at those first and see what kind of frame those mount to easily.

https://www.amazon.com/Harbor-Freight-Predator-Engine-Engines/dp/B07M7FXJJK

https://www.gasbike.net/products/212cc-ohv-predator-engine-horizontal-shaft-100-cnc-bicycle-mounting-kit-free-shipping

I'm curious what you come up with, I'd like to see someone put one of those Predator 212 Ghost engines on a frame.