Change the set-up of the hydrostat - higher gearing - faster mowing. 
I have read up that the hydrostat cant deal with the higher speed and you need to use a normal box or summat but remove all the gunk from inside it.
Hydrostatic gearboxes = nono! It will fail very quickly. Besides the oil would overheat and you'd lose drive.
Ours were manual boxes i.e. select the gear then release the clutch/brake pedal.
So for instance if you want to mow (or race) flat out then select 5th gear from standing, then release the brake/clutch pedal slowly...
Swapping the pulleys over front to rear will result in much higher gearing = much more speed. drive belts will fly off though
also the standard hand throttle is a pain, they will usually only rev to 3k rpm. Attaching a cable to the governor and rigging up a foot pedal (or if you're pikey then a piece of bailer twine onto the governor and tug on it) meant these things will rev their t!ts off. Lots of smoke and backfiring = annoyed neighbours.
So:
Pulley swap = (approx 4x speed (assuming circumference was double on the larger pulley))
+
Holding governor open (= approx 2x revs)
...would result in around 8 times faster top speed and LOTS of wheelies.
These things do not corner well either, unless you mount the seat much lower down to reduce your centre of gravity! You
will fall off if you use the standard seat.
And brakes? What brakes? they have a single teeny weeny brake disc on the rear axle, which is about 1.5" diameter! A friend of ours went through a barbed wire fence at full speed and if it wasn't for his downhill biking bodyarmour/helmet/goggles (yeah we mocked him to start with) he would have been horrendously scarred!
