I have the same bike. It takes a 21.15mm or 0.83" stem. There are some cheap stems available new in this size or you might find one on ebay thats better quality. This 1 1/8" adapter will work nicely if you don't mind the look. Lots of modern 1 1/8" stems will fit it. https://www.ebay.com/i/321296124414?chn=ps
Or if you know someone with a metal lathe that knows how to use it you could get something like a Nitto Technomic or a Technomic Deluxe and have it turned down from 22.2mm to 0.83". Thats what I would like to do but I can't find anyone to do it. So I'll probably just use the threadless stem adapter mentioned above. Maybe you could sand a 22.2mm stem down to fit but thats a lot of metal to remove and it might be hard to keep it round.