I am not certain whether you will locate a burner suitable for retrofitting, but I don't know whether it would prove out in the long-run. The burner gave out on my Bowen water heater in the '64 Overlander in 1998 - - at that time my dealer was able to find the part, but it was about 1/2 the price of a new tank and there was evidence of corrosion around the tank pointing toward the possibility of pinhole leaks - - long story short, replaced the water heater with a new atwood and am quite pleased.
The only down-side to replacing the water heater is that the new unit will be smaller requiring some patch panels be installed, and you will need to make a decision whether to maintain the '64 esthetic and modify the original water heater shroud or go with the new one that comes with the water heater. Since my original furnace shroud had already been damaged and removed, I chose to go with the new water heater and its door - - I am still trying to get around to painting it aluminum so it doesn't contrast so much with the side of the trailer - - one of the advantages of my Minuet - - its new water heater doesn't shout that it is new.
The following is a photo of the completed installation before the exterior was polished and plasticoated:
There is another photo in my album that shows the front half of the door after restoration, but none that really show the installation well after restoration.