I have a hydro murray , pulley on it is already like a 3 1/2 - 4" pulley - you couldnt go any smaller anyway.
Id suggest researching on building a modifed tractor , after my first tractor failure, i started looking at different setups , im not saying the second one was any better, i just didnt apply anything i learned tward it - that almost bot me more then one trip to the ER.
My future builds will be a whole lot safer, it might be fun to make a plain tractor fast ( i felt that way at first) , till a crash sends you to the ER- i wised up before that happened.
Now i think back, my first mod was scarey as my second, however the 3rd was a whole lot better by simple changes that actually cost me nothing to do and was still basically stock - lowering is a biggie- lower center of gravity makes for a more stable machine - can do this by smaller wheels, lower profile tires, lower the seat position ( might have to cut the rear fenders) - i bolted my seat right to the frame - front axle bolted tight- no swivel.
The difference was remarkable, it handles so much better sitting lower on the tractor, could throw it into a corner and not worry about flipping it over.