The Thunder Tiger 46 Pro is a very good engine, the Super Tiger is supposed to be, but I've heard a lot less about it. The GMS is an iffy proposition.

The OS 46 FX would make the Sportster a real screamer. I had one with an older OS 40 FP in it and it flew fine. I've also owned one with a 52 four stroke in it, and it has better vertical. I flew a club members with a 46 FX in it to trim it for him, and the plane was a rocket. Don't have to worry though, they slow down pretty well for landings.

The RCU critics rate the OS 46 LA as a 50/50 engine. About half like them, half don't. The one I own runs fine and gives okay power for a low end engine, but if you can swing one of the better ones, I would.

Good luck and have fun!
Dennis-