Just a wild thought here, Maybe you can find a comparable OS carb to to fit in your ASP 91? I'll bet you a million that the stock carb is to blame for your troubles.

If you just want to get a new one for cheap, occasionally you can scan the RCU for sale ads, and find some great deals on used 4 strokers. Thunder Tiger also has a four stroke line, I have been surprised with their quality, and user friendliness, The TT 91 would be a good choice. Magnum also has a 91, maybe its too similar to your ASP, But 8 years later, They probably fixed the problem you have had, and are running great now??? Anyway, worth a try, you would still have a new warranty, and if You hate it too, send it back!
Two strokes are another option for your CG Extra, I think the ASP 108 would be good, Very powerful for your plane, would give you great Vertical. Or Super Tiger's 91, its a cheap engine, and pretty powerful too.. And then Thunder Tiger may make a 91 two stroke, I cant remeber, but its worth a look.

There are lots of cheap alternatives, Search around, You'll find one you like. And remember, You'll have a Warranty..

Hope this helps, Coulter