The carb barrel/venturi opening of the TT .42 GP is only .220" IIRC and I think if you were to open that up a little...say to .250" perhaps, that would be a good start. (that and a better muffler, maybe the GMS .40 muffler would work? )

The carb is a bolt on type, and that makes it virtually impossable to swap carbs...unless I suppose you wanted to machine some sort of adapter.

When I modify a sleeve, all I typically do is to radius the lower lip of the port openings on the outside of the sleeve...and I have seen 200 to 500 r.p.m. increase depending on the engine. It also depends on how efficient or (relatively ) crude the engine is in the first place.

I got 400 r.p.m. more from my O.S. .61FX for example with the sleeve mods and a little polishing of the crank.