When you build up from the action be sure you build it to what the final modification marked on the action is. Do you have a supply of Mk1 barrels for Mk6 ammo. If you are just screwing in SC barrels then any mag is correct . The Brits ,Aussies or anyone else were not concerned about what came out had to go back in the same. Parts are parts. Modified mags are a collection of parts.
No matter what you do the rifles will still be Bitzers not FTRd rifles .
Do you know why the rifles were stripped out, do you have the correct gauges to do a FTR . If you can not say yes to the gauges don't attempt it for a shooter. For a non firing wall hanger go for it.