I never thought of it at the time, but as the Banshee rotor comes with a trigger, there's probably no reason why the Banshee controller couldn't be made to work. They are cheap - around $50.
There is a whole lot of stators that can be used from Honda to Kawasaki, Suzuki and Yamaha, information which ones have the right dimensions is on the web. Not sure about rotors, the Banshee rotor has the same taper, but I'd imagine there are a few others that will also fit. Be aware though not all are priced the same, so it may pay to shop around.
I prefer new parts as you don't know what you are getting second hand, and of course, why put 30 or 40 year old parts on your bike, you are trying to upgrade it, remember??? New parts for most bikes are available from China. China stuff is rubbish you say??? Well, where do the the XS650 parts suppliers get their parts for their kits from you think? Pretty well everything is made in China now, like it or go without, why pay the middle man?
The Banshee mount can be modded to suit, but will just look like a bodged up fitting, yuk! Again many of the XS parts outlets stock mounts. You can make one yourself as I did, or get your local machinist shop to turn one up. Just be aware, as the rotor is magnetic and the stator base is steel, the stator has to be mounted in the right position relative to the magnetic pull of the rotor.
The diameter of standard woodruff keys doesn't fit the XS650 crank slot well enough, so as well as stepping it, you'll also have to file the diameter down, either that or make one yourself, as I did.
Then there are rectifiers and regulators. You can use separate units, three phase rectifiers are available on ebay for around $6, and I believe a Dodge regulator can be adapted. There are also combined reg/rects, which I used, how much you pay is up to how well you Google ebay, prices go from around $25 upward, pretty much for the same thing, I paid $25 and it works fine.. I'd bet they're all made in China anyway. But you'd be better off sticking to a reg/rect that's for the same bike as the stator.
Despite all the work it's worth it to get a decent alternator. Best mod by far. Well if you don't count, cable splitters, single points, VM carbs, ally manifolds, coolers, remote filters and so on.