An inverted 91 or 120 will be fine. Probably the 91 would be on the low on power. Also, you might want to get an on-bord glow ignition as inverted engines sometimes flame out at idel. I've inverted serval engines and haven't had any problems. Don't let people tell it can't or shouldn't be done. Nothing looks as awful as an engine hanging out in the breeze on a scale airplane.