OS 120 FS on mine
I have one with a 15 year old OS 120 on it. I fiberglassed and painted the plane and the kit's already fairly heavy. Not quite enough power to hover in this configuration swinging an APC 15x8.
The plane's incredibly overbuilt. I wouldn't worry at all about the 108 overpowering it, although I'd heed the above warning about prop clearance. This plane ain't got much.