I'm using a YS .91 that is well broken in, and inverted. It's a good engine for inverted flight so that is not an issue. As for power. It will hover well and it has enough power to pull out of a hover - barely! I'm running it with an APC 16 X 4 wide blade prop on 15% Cool Power and it weighs a tad over 7.3 lbs. More nitro would help, as YS engines really like nitro or I could go to a bigger engine, but I've already got 3.24 oz. of lead in the tail to offset the YS. I put the fuel tank right on the CG, which is the one specified in the book.