For belt drive frames, you'll probably be looking at the touring internal gear hub market. STEEL? Although, I'm also seeing a couple of fixie bikes for sale (also steel, I think).
They should be designed for tightening the belt, so it should be easy enough to convert to single speed.
It looks like Gates sells a 19T and 21T threaded "fixie" sprocket.
https://www.gatescarbondrive.com/pro...rear-sprockets
There is also a 22T freewheel, and I think the 9 spline (20T) should work on a Shimano style freehub????
Hmmm... Do you want "Single Speed" (freewheeling), or "Fixie" (track style, pedals move with the hub)?
There are several brands of "single speed" freehubs. For example Hope has a single speed "Trials" freehub. I think STANS has a little cheaper hub. Some use spacers and a standard cassette lockring. Others are threaded somewhat like the vintage hyperglide freehubs.
Once you find the rear sprocket, use ratios to determine the size of the front chainring.
So, say you choose a 20T sprocket. And wish to have something similar to 55/19.
Try:
55/19 = X/20
X = 55*20/19 = 57.9
So, about a 58T ring.
They've got some big rings, but you would have a choice of 55T or 60T.
https://www.gatescarbondrive.com/pro...ront-sprockets
130BCD will determine your crankset.