You’ll probably have to buy a replacement neck! Musicman will probably sell you one!
Are you sure the truss rod snapped? Sometimes the nut breaks before the rod does and it could appear like the truss rod snapped but all that needs to be done is a new truss nut needs to be installed. That repair can be costly but it might be cheaper than a new neck. Someone with access to an X-ray machine could actually find out the extent of the damage