Transaxle gearing Differances?

I have a SS/twin 16, My orignal plan was to just part it out. i have now been convinced to build a puller out of it..the burb does not have the original twin that came with it. I do however have ACESS to a kohler k 12. my question is, is there a gearing differance between smaller larger hp engines.. if so is it internal in the transaxle or is it as simple as different pully sizes.. i dont want to put a smaller engine on this machine and it be geard like a pig any imput is appreciated

thanks
Kyle