Ted the forks have to be Pre78 the same as the frame, engine, wheels, brakes, swingarm etc although the internals of the forks are free to do as you like. The same as the engine, if you can somehow fit a 2013 RM 125 engine inside the cases of an RM125B without changing the external appearance go for your life, granted a little difficult with the water cooling. If you can fit the internals of 50mm forks into the RM125B forks without altering there external appearance go for it as well. The rules are reasonably clear in that area the same as your alloy swingarm really. After market adjustable shocks were available in 1977 and the RM125B had remote reservior shocks standard.
The RM125B parts book was printed originally in 1976 but no one said the swingarm was built then but that is not the issue and never has been. The contested point is that the rules say the entrant must be able to prove eligibility. There in lies the problem with the swingarm to date no one has been able to or confident enough to front up with one fitted with the 'proof' it is eligible in case someone protests or the scrutineers rejected the bike. I think it is legal but I am not going to go to the trouble as I don't have one and don't intend getting one.
I put together the case for the VB series Montesa and also the '78 VB as a carry over model and submitted it to MA and it was accepted about 3-4 years ago. The system works.