What you're calling 'pre-alpha' is infact a very old 'O' drive. (there is no such thing as a pre-alpha drive. 33 different drives fit the description as 'before the Alpha One').
It may bolt up, but the shift system is incompatible. I have looked on my Merc part drawings, and also a variety of websites. Nobody is showing breakdown drawings or listing parts for the old 'O' drives any longer. The overall ratio of the 'O' drive for the 90 was 2:1. Close to the 1.98:1 of the Alpha One, but I have no idea how the ratios are split between the housings. What I mean, is that the Alpha upper is 1.65:1 and it's lower is 1.64:1, giving the total ratio of 1.98:1. Without knowing what the tooth count in the 'O' drive lower is, no idea what the final ratio will be.
Chris.......