I assume you need to replace the transmission drive belt? If so, you need a 1/2 inch by 84 inch Oregon Type A 75-484 belt (plentiful from ride-on mower dealers). Jack up the machine. Make sure its safe before you get underneath. Undo the tiny split pin securing the gearchange arm under the frame by the rear jockey pulley. Move the arm out of the way.
I found it easier to remove the guard plate under the engine drive as well.
Now slacken off the Jockey pulley bolts and ease the belt guards out of the way so you can get the belt out. You don't need to take the pulleys right off.
Remove the seat. (Now you need two people).
Undo the 4 bolts securing the power take-off drive plate from under the seat void. Don't lose the spacers. Looking from the rear of the machine, note the 2 right hand bolts are longer (they secure the moveable tension plate). Once removed, lift the whole pto assembly off. The spring on the moving tension plate need not come off. Just park the plate carefully. The base plate separates note.
From underneath, wiggle the old belt off the rear drive pulley on top of the gearbox. Yes, it will come off. Pass it clear of the three Jockey pulleys and remove it from the engine drive pulley.
Refit the new belt in the reverse order but pass it around the two FRONT jockey pulleys first, before putting it over the gearbox drive pulley.
Reassemble the rear Pto and tension plate and use NEW Nyloc nuts. (If you don't know the bolt size, look on the Web at Westwood Spares - they have a superb parts list and diagrams. It is likely to be a 5/16th UNF).
The two locating lugs underneath the pto should go back in to the holes in the top of the gearbox drive. Make sure you insert the spacers on the 2 RH bolts the right way up - reduced collar uppermost).
The upstand belt guard should be tightened so it is not too close to the pulley face, as you need to get the power take-off belt to the sweeper through it.
Now pass the new transmission belt over the rear jockey pulley. Make sure the new belt is central on all the pulleys.
Replace the gear change rod and split pin. Replace the guard plate under the engine drive pulley.
Check the tension on the belt by moving the clutch pedal. If too tight, slack off the collar on the rod right at the front of the engine bay, it may only need one or two turns. DONT over tighten or over slacken it.
Replace the seat. Check the new belt is not rubbing against any projections and the jockey pulley belt guards are in the right place.
Run the mower and check the belt and pulleys again.
This operation took me an hour and the belt cost £31. At the same time I took the opportunity to clean everything underneath. My machine is 28 years old, gets used heavily and is still immaculate.