These old mitsubishi cdi boxes had 2 problems. Moisture and cracking solder points. 1st thing you need to try is to remove the cdi box and place it in a pre heated oven at 275° for 15 minutes. This will bake off any moisture inside without damaging anything. Next, if the bike runs great, seal up any cracks or vent holes. If it is still not running good you can repair it by gingerly splitting the case with a hacksaw and re soldering any cracked solder points. I know its tedious work but its worth it.once inside the box, you will encounter a gel coating that can be removed with acetone, make sure to use gloves and eye protection and let it soak gel side down in a 1/2" of acetone. After 8 hours or so, you should be able to gently pick chunks of gel away and expose the back side of the circuit board and solder points

Many thanks Bob for the suggestions. I have the same bike as the original post, a 1982 Suzuki GS300L, and the same issue, a failed CDI box. It's discontinued and hard to find, so I'll open it up and see if I'm able to repair it. Fingers crossed!