First, welcome to OCC, Hober43!
If J-Mech is right and it's the pin on the trunnion shaft inside the hydrostat, that's a challenging repair. Let's hope it's something else in the linkage from the dash to the trunnion arm on the outside of the hydro. You'll need to operate the lever and watch the linkage for any play or damage. Probably best to put the rear wheels up on jack stands. There should be a set of springs like the pictures below on the lever operating the hydro. A small spring inside a larger spring with a pair of metal end caps.
If the springs are there (inside that shiny metal rectangle-see pictures) and there is no obvious looseness in the linkage, then it could be the trunnion shaft pin and the tractor is actually dangerous to operate as you may lose control of it and not be able to stop it. Hence the jack stands...
Seems to me the trunnion shaft pin is a low probability failure, so look elsewhere first.
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