RE: YS 1.10 vs. O.S. .91fx for Funtana 90s?
I think you'll be happier with the Y.S. engine because of the extra power you'll need at your altitude and i'm not sure if you need a pitts muffler for the Funtana but if you don't need it then don't bother buying it, i'm sure you'll be happy with the stock.
I've seen a Funtana with the y.s.110 and i know it's a great combo but i'm not sure about the o.s.91, to me the .91 just barely meets the requirements where as the 1.10 will be better suited to give you the thrills you're looking for.
Good luck : )