When I replaced the 20 year old JIS no-name headset on my World Sport a few weeks ago (I ran it for a year with loose bearings before it was no longer useful) I didn't know that I could still get a decent Tange steel JIS headset. In hindsight, that's exactly what I would have done.
I bought the cheap nashbar threaded headset, punched off the cups and races with a long piece of all-thread rod (gently, but it didn't really matter because I was throwing away the old hs) or a punch. I don't think I'd ever do it with flimsy aluminum cups.
I also gingerly filed down the JIS fork crown race to ISO size with several grades of files and emory cloth until I got an absolutely perfect (feeling) fit, but maybe that's fodder for another thread. Honestly, to do it all again, I'd either replace the rather crummy bike or just get the proper sized headset. It does work like a charm, but it was an awful lot of trouble for a mediocre (at best) headset on a low end bike.
[note: I only added this because I didn't find, either searching here or the internet, any chronicles, either successful or otherwise, of a DIY low-budget fork crown machining and I thought this might be of interest to others.]