IIRC,the Euro standards for demilling do not count in the USA, the BATFE views the receiver as once a machinegun, always a machinegun. I assume you mean it is welded, barrel filled, firing pin removed, etc? Well, the BATFE says it can still be repaired!
If it was "Deactivated" as specified by the BATFE, then it is no longer a gun and can be imported; the receiver is "the gun" in the eyes of the BATFE; however, their definition of how a Bren needs to be dealt with is fairly aggressive and involves torch cutting the entire gun in three pieces, with the cuts rather narrowly defined as to where they must intersect & destroy certain key components, and require a minimum of 1/4" of metal to be blasted away from the cut sites using the gas axe... Basically, it won't be capable of firing ever again. You can possibly remove the receiver and barrel, leave them in the country of origin, and take the rest of the parts, mount up a dummy barrel and receiver, and have a nonfunctional display replica, but that's about all. If you are serious about importing this I advise contacting the BATFE NFA technical branch and confirming what parts are allowed to be imported, to save yourself a trip to Federal Pound-you-in-the-@ss prison.
IIRC there are Bren parts kits already in the USA that are legal. Check out Inter Ordnance.
IIRC Historic Arms LLC was making Semi Auto Brens from parts kits, there are quite a lot of parts that need to be bought including a new receiver. Not cheap.