ALL I have been asked to look at in the field has been a combination of pre-alphas made from mismatched drive parts, gimble ring to gimble housing and numerous other combinations and modifications. In fact I have NEVER seen an unmodified pre-alpha and it would be hard to call them their true name because they truly are not what they were when they started their life. (Useful mainly) and almost one hundred percent wanting to be replaced...
So this is why I'll always refer to them as pre-alphas. The only reason they are even around is because they aren't used in any meaningful way. Most are mostly obsolete and the mint ones are owned by those who can afford not to use them.
The only 'mismatching' is of major sub-assemblies, and Merc made a lot of sub-assemblies backwards compatible on purpose. If you have an old MC-1 drive that has a destroyed gear housing, you can put a new gear housing on that is from any one of R, MR or Alpha One. Same the other way, and the same between the later MC1 transom assemblies and the R, MR and Alpha One.
Swapping them around is very common, and very acceptable. If you are buying, then you just need to be a little savvy about knowing what you're looking at, and how to determine gear ratios. And it's why Mercruiser is usually the choice of people trying to built 'cheap boats'...
Merc made a move away from the whole family of the first 'Alpha' series (MC1, R, MR and Alpha One) in 1991 with the Alpha One Gen II. No interchangeability of any parts between the previous and the Gen II... And people got upset. When they found a local reef and wiped out the gear housing, they could ONLY put a Gen II gear housing on, and they were expensive! I have seen one attempt at putting a complete Alpha One drive on a Gen II bell housing. It was on, but it was messy, very messy. A lot of chopped up components!
I hope they are gone within my lifetime
Unlikely, as people just keep repairing them (playing 'mix and match' if you like). Why spend the best part of $6,000 for a new transom assembly and drive when a new gear housing can be obtained for less than $900?
I'd also like to see the old points and carbs gone, but I'm aware that it's not going to happen either, because people like them (it's beyond me why. They say because of 'reliability' or 'ease of repair', but from my experience, both as an owner of old carbs and point and of MPI, and that of maintainer/repairer, the new MPIs are far more reliable than carbs and points, and thus don't need to be constantly 'fixed'), Looking at you
@Scott Danforth 
forums.iboats.com
)