The original filer tube was 1 1/2" aluminum tube with a brass cap. A standard brass sillcock was threaded into the brass cap, and there was a Schraeder valve on the side.
I suppose you could have a machinist make the cap. If you want the dimensions I'll be happy to measure mine and send them to you.
The most difficult part to reproduce will be cast the trim ring that surrounds the pipe where it passes through the outer skin. I don't know where these came from. You might try Colaws or some of the other boneyards. It's probably common for these parts to be tossed when people switch to plastic tanks because they no longer need a pressure-worthy system.