There is no legal way to convert an un-papered machine gun into a legal Title 1 firearm. Merely being in possession of the gun without the appropriate tax stamp is technically a federal felony.
Sometimes people have wound up discovering war trophy machine guns when a relative who was a veteran passed away. The proper thing to do is to contact the BATFE and arrange for disposal of the weapon, assuming that you came into possession of it innocently. They might be able to arrange to have it donated to a museum.