Is it possible to take a 2.25 early 90 engine and gear box and fit it into a series 2A without majour mods to the mounts? Also will the gear box fit and couple up to the drive shafts as the gear box is diffrent in the 90.

Thanks for the help!

dunno, why not try it then you can say yes or no when someone else tries it

Engine will fit no problem, it's basically the same apart fron 5 bearing crank like late series III.
The problem you will have is if you put the gearbox in with the transfer box, that will give you permanent FWD. The Series front axle cannot deal with that because it has UJ's not the CV joints of Defenders.You could use Stage 1 V8 front axle that has CV's. It's possible of course but you will have to change most of the drivetrain. plus I think the gearbox mounts are different.
Plus any axle change apart from Stage 1, you've got the suspension to deal with as any later axle is set up for springs not leafers.