Pretty much the ONLY engines being used in RCCA/AMA Slow Survivable Combat (event 760) are the Magnum .15 XLS and the OS LA .15 This is partly because there is a maximum price limit ($85) on the engines allowed, in order to keep the cost under control, AND in SSC there is a prop and rpm limit, so using high performance engines won't do you any good anyway. The Magnum .15 easily exceeds the maximum allowable rpm limit and has to be throttled back anyway, so why use a $100 + engine when you can't take advantage of the performance anyway? The OS 15 CVS would not be allowed in an official contest because it is WAY over the maximum price limit. But for your application where you aren't using it in SSC competition and you need the power due to the altitude, one of the more powerful .15's or perhaps some of the .19's and .21's would fit in your cowling.