Ross, in addition to acquiring a new or rebuilt drive shaft, I'd like to pass on an observation from John Lang (pretty sure it was him) from many years ago.
Inspect the stub shaft that bolts to the engine. Make sure it is still round and not under .999" in diameter. Bad u-joints can cause a lot of wear on that shaft. The u-joint bore is a snug slip fit at the front to the engine drive shaft, as well as the key. This is to allow slight fore and aft movement of the u-jointed shaft due to frame flex, engine torque, etc.
On my original shaft as well as the old style replacements, the u-jointed shaft did not reach far enough to engage the full length of the key.
The bigger problem is that makes less than a 1-1 length to diameter ratio of shaft support. In plain English, the 1.00" nominal shaft should be in the bore at least 1.00" deep.
The new shaft with the grease fittings is the same length as the old shaft. To make the u-jointed shaft locate farther onto the engine drive shaft, I made a spacer that locates between the flywheel and the shaft using longer bolts. There may be longer engine drive shafts, or spacers available some where to work with the repower engines (Kohler). Hopefully someone on here can provide that info.
Any spacer added must be precision made to pilot to the flywheel and shaft and symmetrical to assure balance.
For what it's worth, my 317 is running the KT17 series I engine. I made the spacer about .400" thick to allow more shaft engagement without risking bottoming out on the flywheel shaft in use. If you can get a new u-jointed shaft made complete, you have the option of having it made longer overall if needed for your tractor.
Some photos that might help with this long winded post.