I used a condensor from a Honda CB500F, 1971, on my Motobecane. I plugeed the hole in the magneto plate where the old, expensive, proprietary condensor used to mount. I mounted (grounded) the Honda cap to a frame bolt near the coil and connected it to the coil points terminal.
Dont worry about 6 volt vs 12 volt, these caps are rated about 150 volts anyway to handle the inductive spike from the coil. If you can match the , capacitance, that is good, but if not, try a lawn mower or motorcycle cap and keep an eye on your points.
I used to know a rule once that was something like; minus metal, minus capacitance, plus metal, plus capacitance. I think it refered to the fixed point. Obviously, this could be either way around so if you find pitting on one of the points, change the size of the cap and see which way it changes. Then post here and let us know. This info could help those who use a replacement coil too.