SO a bit of a resurrection, to talk technicalities. BSS325i has been hugely helpful but he is (thankfully) incredibly busy these days so I thought I would bother everyone else
Engines: I am trying to get to the bottom of what Alpina did to the 2.5 E30 M20B25 engines.
I know the early C2 2.5 were actually a 2.6 (2554cc) using the B23 crankshaft and block bored out to 84mm and these had to have used custom pistons.
The later C2 2.5 and then the C1 2.5 were based on the M20B25 325i engine using motronic, however I have a sneaky suspicion that they left the engine internals alone. It was just exhaust, cam, chip, throttle body, mild head flowing. Pistons, crank and head all standard. (the capacity is the same as the standard engine 2494cc) matching the earlier output of 190bhp
Trim I thought the seats that were in the car when I bought it in the 80s were from the E21. Turns out they are not, they are E30 Recaros but with the E21 broad stripe trim. I now see other E30 C1 2.3s had this trim. I still don't like it (which is one of the reasons I took them out) but at least I am sure they are the seats it should have. The trim is shot (other reason I took them out) What they get re-trimmed in I have not yet decided but the E30 thin pinstripe feels too modern for an old Henna Chromie.
My best guess is that sometime between '84 and '87 an Alpina C1 died and they used a 323i 'sport' to put all the bits on. By '88 it was Alpina'd and it is looking like whoever did it made a proper job of it too. I have tried to find out which car died but I don't think I will ever know for sure.