Hi, I think I can help you with this. On top of the Hydro unit is an L-shaped tab, it should be just beside the cooling fan. The short end of the spring hooks on there. The long end hooks onto the footpedal assembly. If you look at the footpedal shaft, as it extends outward, there will be a plate that fastens to it that extends backwards.It will have a long slot in it towards the back and three holes in the front. The first hole is used to attach the pedal to the plate. The spring should attach into the bottom hole to the rear. It's easier to attach the short.end first. If you have any trouble, don't hesitate to ask, thanks, PK.
I think what you have is the spring for the dump valve. The ones on the back are for the brake. If you follow the link below it'll be #63.
Let me know if that's not it.
I'll have to see if I have a similar mower in the shop and take a look. I can't think of any other springs that size which would affect the drive. Did you verify that the brake springs are in place?
There is one other thing to check and that is that the drive Idler has a spring on it. The spring in the diagram doesn't match what you have, but stranger things have happened....