Any radiator repairer can do that job - there is nothing special about a Morgan radiator.
FWIW I have a heat exchanger on my boat - it is in effect a radiator but instead of air flowing through it, it has sea water. Before I bought the boat in 2009 the owner had bodged a repair of the stack of tubes in the heat exchanger by using epoxy resin. To my surprise, that reapir still hold water and pressure tight today. Seems to be every bit as good as solder.
