xs400's are brushless where the 650's have brushes!
Yeah, I meant that they are the same in that they are both basically electromagnets and not permanent magnet type. They both need power to make power.
I don't think it'll make a difference if there are brushes or not as to whether the cig sized battery will work.
If you're really thinking about not using the starter, and really want to get rid of the battery all together, see if you can find a way to convert the charging system to a permanent magnet type charging system. If you can find a permanent magnet rotor (flywheel) from another yamaha that will fit the crank, it should be possible to do.