You might want to check around for a replacement differential, just in case, but I imagine it might be hard to find. If the bearing/s have failed and released metal into the oil, the other bearings might be damaged, as well as the gears.
I’ve seen lots of ads for banjo style rear ends (and parts) in hot rod mags. Replacements just might be available.
That’s good to hear, they last (mostly) forever, only way they go away is if they’re scrapped/recycled. Replacement is 100X easier than repair, in my o-pinion… 
That’s got my vote. R&R of the assembly is definitely easier.
A Pontiac chunk may be quite rare. If the ring and pinion gears and carrier bearings are OK the rear end can be disassembled and reassembled if the parts are carefully marked for proper positioning. With some patience you should be able to get things back together.
You might go online to Desert Valley Auto Parts. I have purchased parts from them in the past. They do quite a bit of trade in classic car parts. I would suspect that a rear end for the 1954 Pontiac might exchange with other years and possible other makes in the GM line. You may not get one with the same gear ratio, but on a car that is not a daily driver, this might not be critical.