I'm looking in to buying an OS 91FX, but first I'm curious about how much thrust it puts out and on what props. Anyone got any thrust ratings for the 91FX?

They usually turn a 14-6 APC right close to 11,000 rpm

Hi!
Why are you intrerested in what thrust the engine puts out! That isn't important for anything but in a hover. Are you going to hover it?
Most of the time APC is the prop of choice...at least for "small props" from 4"-17" diameter.

I'm wanting to put one in a Fliton Inspire 60 and I fly at a high altitude.

Since were at the same altitude, Your 91fx is going to perform more in the 60-70 size range. If you want it to come close, You'll need a pipe or ultrathrust muffler