a man sized project. Can you turn the spindle? its probably hardened. If you made a new one, not a small undertaking, at least you could revert to ithe orginal t if all else fails.
Maintaining concentricity between the bearing surfaces and the collet mounting will be a bit of challenge, ideally turned on the same set up. turn the od and and then the ID with a fixed steady?
Definitely go R8 over morse. The angle of the R8 is semi self releasing whereas you will have to take the drawbar out and wack away at your tooling to release a morse taper, plus tooling for the R8 is readily available. Think if you torqued down a centre in the lathe with a drawbar how miserable it would be to change extract it.