I know on the A15 the mounting locations on the bottom right of the block are in a different position to that of the early A12 and therefore will not allow a simple bolt up of the early bracket. I would assume the 120Y A12 would be the same as the A15.
Also depending on which head is on the block might determine if the alternator adjusting bracket will easily bolt up. I dont know if all heads have this location for the bracket.
All are worth checking out.

ok thanks for your help fellas

I think you could also swap the brackets and mounts to the new motor, the bolt holes should all be in the same location. Then, you might not have to do any wiring or fabricating, maybe...

The spare engine you have, is the later 120Y style A12.
It just means that you are going to have to look it up in the Tech Section, as this engine has different engine mounts. They arent hard to make.
The alternator is easily wired up. You can just strip the wiring back (remove all the tape) and run it to the new location. Its easy since the wiring is coming from that side of the car.

hey my A12 that is currently in my wagon has the Alternator located on the right side of the engine (if u r looking at it when it is still in the car) and i have another A12 to put in, but the alternator is located on the left hand side why is it like this? is it just a case of the age difference? thanks