The key that is left is too short. At one point it broke in half. They had one 5mm key that covered both sprockets, at least 30mm long.
I'd bet that Coleman sold a lot of those jackshafts. In your case the jackshaft (not the assembly) and a new 5mm key would get you going. The sprockets may be ok. The keyway on their 16mm jackshaft is what gets beat up.
But the next thing to go would be the bearings if you don't replace them during the repair.