Coker Tire seem to have a couple of options - including some in what they endearingly call "bias-look radials" that would seem to fit the bill. One possible answer is linked below... I've used Coker before, but am not at all familiar with "excelsior" as a brand. But it's a place to start looking, perhaps:

https://www.cokertire.com/tires/styles/bias-ply-tires/excelsior-vintage-european-style.html

One of the size options is 500/525 x 16 which I'm guessing would be what you need

Doug
1985 4/4 4str, Jubilee Blue