They are most popular in the 39 and 41 mm choke sizes.
41s would be too big for a 650. Even the 39s would be a bit large, but should still work if the engine has decent compression. I tended to recommend the 39s for road use, even on the 900. (The 41s are more a flat-out track arrangement).
The other variables are that they come as a kit suited to fit different models of engine, including the twistgrip and cables - Ducatis or Yamaha TRX being the most popular. You are probably going to need ones which operate individually, rather than the more common paired 'bank' style, which operate on a common shaft.
The intakes need to be the long ones to fit the paired type, so you probably need to park your bike alongside a later 2-valve Ducati with the tank up and work out whether they would clear the frame.
Fitting the later intake arrangement, and an airbox, may present a challenge.
I'm guessing it's not going to fit on a Pantah frame. (A TT frame would be fine - not sure about the tank). So you will be looking for two individual 39s (or smaller if you can find some) if you still have the stock Pantah frame..
They will need to have the pumper mechanism on both carbs. The paired type only have the pumper mechanism on the LH carb, so you can't get a linked pair and separate them without a fair bit of hassle.
Maybe just get it going with the Bings and look for alternatives at your leisure, without being off the road in the meantime.