Points, Have you checked them? They need to be clean, gapped and timed right.
You have two sets of points. One for each side. The side not running right is the one to check first.
New points are not expencive. Around $20. Your repair manual explins how to do this.
If that don't help it may be the coil. Both the primary and secondary sides. A multi-meter is your friend. Checking all the connections between the battery and points All these connections should be cleaned and tightened, right along with every othe connection on the bike. Start at one end and work your way to the other. A bit of dialectric gease will help keep thing clean.
Leo