Well, thats the whole problem. Sinistar only works with an analog stick. But... the stick the arcade used, was special in a few ways...
http://epg3.tripod.com/sinistar.html(look far doen the page for pics and an exploded view of the joy)
1) It was a 49way optical (no biggie, as an analog stick works almost the same)
2) Its centering was done with the rubber x shaped 'spider' as they called it. This makes a load of difference in the control and feel of the stick. First off, its centering was perfect with no 'play' at all. This means no accidental drifting when going slow and tring to pick up those crystals. Also, when you move the stick, you are fighting 3 out of 4 legs of the spider... and the further you go outward, the Harder it is to push.
A normal analog stick, uses a centering spring. When you push outward, you get a 'constant' force to return -(not exponential like the original) ... so its hard in sinistar, to control your speed well. Being that a lot of joys. centering isnt stable, you get unwanted drifting... and, because the joy is so easy to move from the center to the very outer edges, you end up going too fast.
I hacked a joy, and made a makeshift centering spider out of some innertubes, and an analog stick. My first attemp was a little weak (still on website), yet finding a sronger joy, I was able to get it going without it falling appart. The control is soooo much better, it almost made me cry! : )
Still, it would be nice to have an more durrable solution... as well as the fact that it isnt usb : ( ugg..