As of July 30, 2010 I was in the same predicament. I have a 38" John Deere sabre with a broken hydro drive belt. How I got the belt on was extremely hard. I ended up undoing the tensioner spring to allow the bigger pulley to slide close to the smaller pulley. Then I put the belt on. The problem was I was not able to reattach the spring.

You need to take off the mower deck. Its just not possible to get under the tractor to get the needed leverage. A much stronger friend came over and undid a belt around the pulley then using force reattached the spring. Then, we had to use a screwdriver to pry the belt back on the pulley. It took two men to do it. Then reattached the mower deck. It took about 20 minutes once he came over.

In hindsight, I would not have undid the tensioner spring and just pried to the belt on the smaller pulley (idler). It just takes massive amount of strength either way if you don't have riser to put it on like at the dealership.

Hi,
About 8' of good quality starter rope doubled on itself and with both ends tied together with a single knot and a large screwdriver should be the needed tools. Hook the looped end on the spring and have someone use the screwdriver as a handle to pull the spring from the fron of the machine while you guide it into place under the machine. This is basically the same system used on the older STX hydro.
Peace,
Clarke