Have you looked at how they convert the Sterling Pistol or the PPS to closed bolt? Might help. The PPS43c is the inexpensive semi-auto that is made in Poland but sold here (with welded wirestock).
I could almost swear that the surplus bull barrels classicarms.something had for their Mosin Nagant sporter project product came off a machine gun with a 43 in the name. They looked reeeeeal nice; i never had the scratch or the time to get the package deal, but maybe you could find one from a failed project or another supplier through classicarms. If you need one in the future.