I'm putting a Saito 120 in mine right now. My opinion is that the 120 size four stroke is a tight fit for this airplane. My issues were:
Overall length of engine
Exhaust routing
Balance
Throttle linkage
Stretching cowling over engine for final assembly
Fitting big fuel tank
Strengthening firewall mounting structure
Did I say "balance"?

I have everything under control now. It's a clean installation. It just took a little engineering and patience.

Good luck.