Ive had a few modifed tractors over the years- ive done alot of research on building them as well- mine were for personal use.
Ive learned a few things - stock and speed dont mix. My first one had NO brakes and i trashed the transmission ( luckily as i started out, not going 40 mph or so..) - built a second one, stock brakes were worthless, was insanely fast ( i never had it wide open) was just too unstable ( took that apart, was scared to drive it) - last one i put lower profile tires on it, locked the front axle ( best for stability), lowered the seat to the chassis - it reminded me of a 'big wheel' trike driving it , but it handled excellent - like a slot car- i could throw it in corners and not be afraid of flipping over, but the speed wasnt there- i took that apart to redo it . Its been the best machine so far.
I suggest you definitely do these mods:
-brakes ( small motorcycle or go kart brakes will work- hydraulic are the best)
-raise and solid bolt /weld the front axle ( NO swivel)
-lower profile tires( lower center of gravity/more stability)
-drain the transaxle of old oil ( if applicable) and put heavy duty gear oil in it ( thick stuff) - the trans will whine horribly ( not designed for speed) - i used lucas oil in my second one's trans
-solid bolt the seat to the chassis
- some sort of bering type wheels up front
- bolt or lock the wheels on ( imagine 20mph and a wheel flies off.....)
And wear saftey gear ( helmit definitely) - i always did