http://www.perrypumps.com/prod01.htm

This is a listing of Perry carbs for different size engines. I had thought about it for my 90 ST. My 45 St's (2 of them) run flawlessly. The 90 is a tad finnicky, but I would bet as much on the muffler as anything else.
The Perry carbs seem to be fairly priced, but I have no experience with them.
I also run all my engines both 2 stroke and 4 stroke on 10% / synthetic oil Morgan fuel. They all seem to run well on it, the 52 and 91fs, 45,75,90 2strokes.

Good luck!