Saito 125, if you are stuck on glow 4-strokes. OS is good as well. I would steer clear of YS, Magnum, and all other 4-strokes. It's a crap shoot with them IMO.

Personally, I would go with one of the small gas engines. Plenty of room in that cowl, very clean burn for your airframe, less expensive fuel, no glow start to fool with, the engine will cost less, and the sound will still be cool (unlike electric).

Opinions vary. This is my humble offering.