If the mortar is legally deactivated meaning that it has a bore sized hole in the tube and rods welded across the bore than it can be rebuilt and registered. If its just got a plug than its in a grey area. DD's can still be registered by individuals by using a form 1. The only problem is that you cannot legally register a weapon that is intact and unregistered. The fly in the ointment here is the lack of any detailed deactivation procedure for these types of weapons in the past. Current regs are fairly clear (as clear as ATF can be anyway) but there are still guns out there demilled to old spec....
That is my knee mortar in the video on the Japanese page. It was built on a Form 1.
The base by itself is a Title 1 firearm (rifle, pistol), just like a mortar cup or M79/M203 receiver.
There are three routes you can go-
1. Repair the original tube. As I shoot low pressure loads, a competent, decent repair shouldn't be a problem.
2. Have a new tube built. The original has some wicked small thread on the base and your machinist will have fun with that. You also don't need to duplicate the rolled lip at the muzzle end or the rifling. Other than that, it is a straighforward tube.
3. Get a good tube from another launcher.
Decide how you want to go with the barrel, but don't take possession of the completed barrel. Form 1 it and then you are good to go.
Just don't break any parts. Spares are non-existent.
BTW, check your firing pin. A lot were clipped by the Navy before they allowed them to come on board.