With MG's once an MG always and MG. In this case technically if you put a legal length barrel on it that would make it a non NFA item. The problem is it would still be registered as one so I don't know how that would work.

My advice is to go to ARFCOM and PM TonyK he knows his stuff and ask him or you could even ask David Hineline.

They would know for sure 100%.

Make sure to let me know what they say just for my personal knowledge.