Standard, normal BMx u-brakes will fit your bike, if its oem brake was a deerhead(xt)/deore (shimano) or a suntour of a similar vintage. If you have something very hott and old and exotic (eg, early Ibis), and it came with earlier versions of Cunningham's rollerbrake, it may have proprietary mounts. By the time Cunningham was licensing the roller brakes to Suntour and selling them under WTB, they were standardized, and modern Tektro u-brakes will fit. But, they shan't be as awesome.

Also, unless you're really cool with a shop, they're not likely to open up a product so you can test fit it on some random vintage mountain bike. Maybe they do it for Fiets-Bob b/c he knows them well, or maybe they're like "oh, crap-- here comes that guy who only speaks in haiku... we'd better let him mock-up whatever he wants on his bike, or else we'll have to call his caseworker again."

I have a Miyata with an (almost) full Deore mt60 group; I love that low-mounted U-brake. It's prolly the main reason I've kept the bike. Enjoy whatever you're riding/building! (what is it, btw?)