I don't know of any sources to suggest other than military surplus places which you may have already tried. I might try emailing a couple surplus places that appear to have genuine old military stuff, not China looky-like replica crap.
The thread could be NPS (straight). The ones I've seen have a stamped/formed cupped cap with a wide hex flange.
A pipe fitting would likely fit the threads at some point along the taper of the fitting, but probably won't provide much of a seal (leak badly) if there isn't a flat shoulder to support a gasket to seal against the can's opening.
So even if you could find a decent pipe plug (brass would do nicely), there wouldn't likely be a wide shoulder for the gasket. That would most certainly involve a short section of a close nipple to fit into the can opening threads, then a reducer, cap etc (or other fitting to provide a shoulder for the gasket), and provide a suitable area for mounting a gooseneck (soldered in probably). Rather complicated fabrication project for a fairly simple part, but a new device would be made to all the safety regulations, and likely leak and/or break easily.
You can bet that there are lots of extra caps around, because they're the most durable part of the can (OK, maybe the handle too). Cans have certainly rusted out from sitting on dirt, or having water in them, but locating the type of scrounger that would have a few caps is the difficult part.
Try a vintage military vehicle/Jeep group-forum, maybe. The WWII vehicles seem to have spots for mounting the cans, so I'd think those folks would be on the lookout for cans to make their vehicles look ready for action, dressed out with genuine vintage accessories.