Forget the hydro - they already come with a 3" pulley on the trans and a fan - youre better off with a manual ( with original oil/grease cleaned out).
Even for just messing around, you still need some saftey built in - like brakes, reinforcing, locked front axle, no slop in steering, lower seating..... unless a trip to the ER is on the list as well.
Ive built 3 mod tractors- only one close enough to do any serious speed was my last one( took that apart for its next phase) - the second went ' suicide fast' , was nearly stock and stock brakes didnt do anything to even slow it down- stock height and unlocked front axle makes them nearly uncontrollable - first one i was lucky enough to break the trans just starting out, not at 30 plus mph....