Personally - if it was my S/C, I would disassemble the whole thing, take the shaft and have it machined to accept one of these:
https://sdp-si.com/eStore/Direct.asp?GroupID=551Alternatively go to Mcmaster.com - Flexible Couplings, and check out their spider couplings
or shear couplings
Both options are cheap - you just need to do some GOOD (i.e. not on your bench grinder!) machine work