The reg and rec replacements we rig up are based on what Pamcopete came up with but we've refined the substitutions some. We've discovered more automotive regulators that will work, and gone to a single 3 phase rectifier instead of having to wire 2 single phase ones together. Here's a thread that covers it .....
http://www.xs650.com/threads/regulator-rectifier.21485/
I don't recommend buying any of the motorcycle specific regs, recs, or combo reg/rec units. They're very expensive and a total waste of money. You can adapt these substitutes for a fraction of the cost and they work just as good, if not better. I also like the idea of staying with separate units as opposed to a combo. That way you can replace either one by itself if it happens to go bad.