Once the bolt face is opened up that is pretty much what you're going to have. If it is now machined for the 378 Weatherby case head, it will have to be a 30-378,338-378, 378, 416 or 460. That's not all bad. The receiver itself can be made to work for any of the magnums. The standard length magnums have a block in the rear of the magazine box and a shorter follower. It's just that once there is machining done you really can't go back to something smaller.
It is very difficult to just get a bolt but I'm sure one can be found in time. Weatherby may replace it but I don't think they will sell just the bolt. Good luck.