[Diogenes]

Is it possible to replace all the parts of an M37 receiver with parts from a 1919 parts kit, including the top cover? If possible, this would allow much greater flexibility in caliber conversions, etc.

Or are there differences to the M37 receiver that would prevent this from working?

 

[MG08]

you might want to look at photos of the two guns- While bolts are similar and could be interchanged, there are other non-compatible parts - like top covers, that would require re-engineering. back plates, recoil springs, feed and cartridge stops, all are different.

 

[glassmountains]

With out going into a lot of detal, all but one conversion (7.62x54R) can be done to either gun the same way so I don't see what the advantage is there. Only the bolt has to be permanently modified/dedicated for 54R so just buy another M37 bolt and you're covered.