A lot depends on what you expect out of the plane. The US1000 is NOT a 3D type of plane, so its doesn't need a 50CC gasser. Also, air/fuel ratio doesn't mean much when it comes to running time. A US1000 running as large two stroke glow engine may run for 15 minutes max on a typical tank of fuel, while a gasser will run 30 plus minutes on the same quantity!
Will performance be the same? No, but again, it comes down to what type of flying you want to do for the type of airplane it is.
Being real general here I know but just trying to make a point.
And 14.7:1 is actually the ideal air fuel ratio for a gas engine(looked it up), but who's counting!
Could not find an ideal ratio for glow engines though...
Thats one of the reason I'm looking to re-power mine, going from an old and tired OS 120 FS (non-Surpass) to a gas engine. As mine is built and has been flying for many years, I'm not too inclined to carve the hell out of the front of the fuse though to make one fit, and in the end, a newer/larger four-cycle may end up in there. Still less costly to run then a two stoke glow, plus it sounds better (IMO).
Bill S.