I've got this on my FS60 and it flies it just fine. The one I have has the silver rocker cover but everything else is the same.

http://www.hobbypeople.net/gallery/210868.asp

I would dump the the .91FX for this. I've literally got approx 75 to 100 hours on this engine and have adjusted the valves twice. When it eventually does wear out, a new piston, ring and bearings are approx $45. I've also got a OS 91 fs and I'll never buy another one as long as I can get this Magnum for half the price and as far as I can tell, the Magnum runs every bit as good as the OS. YMMV.

Edit: The OS does look better on the outside than the Magnum.