Well, I used a Briggs and Stratton 8.75HP on my old Manco Vortex. That originally came with a 6.5HP Robin and I freakin ran that engine into the ground! The extra 2.25HP really made a difference. I was impressed. If you have a bunch of money to blow, I know Robin makes a 13.5HP engine, and I can imagine that thing is ridiculous. Heres the Robin
http://r o b i n a m e r i c a.com/series.aspx?sid=9 and heres the engine I bought
http://www.b r i g g s a n d s t r a t t o n.com/engine_power/engine_details.aspx?pid=88