I'd originally planed on using my spare Saito .91 in my Ultimate... until I actually received the plane

It could be made to work. You'd need to add weight to the tail, and would have to hack the cowl to bits. Ground clearance with a 14" prop would make landings interesting - especially once the soft stock gear spreads a bit.

IMHO its far too lightly built to absorb that much power. Nor does it need it. Try a YS 63 or Saito .72 4C; on the 2C side an OS .50 (ringed) is a good choice.