This looks like it would be easy enough to 3d print and maybe worth trying it. Is this a to scale drawing? If you think this is correct if you send it to me, I can model it in 3d and print it out of a durable material for proof of concept.
Do you cover the splines when you run it without a prop shaft? I think the bearings are shot in ours but I do see how the splines could get worn out. I was considering making a support for the shaft that just bolts on each end but that's only if a new shaft does not fix it. Not sure what else...