The bolt on the lever is threaded into the lever. Perhaps the bolt or hole is stripped. Putting a nut on a longer bolt may not help much unless you drill out the threaded side. I might drill it out and thread it to the next larger bolt. You will need to drill out the unthreaded hole so the bolt slips through.
Perhaps the splines on the shifter lever or the shaft are stripped. One way to fix this is to drill a small hole, like 1/16 or so. Drill this hole at the place where the lever meets the shaft, half the hole in the lever, half in the shaft. With the bolt loose drive a roll pin into the hole you drilled, tighten the bolt.
This will get you by until you can replace the lever and/or shaft.
Leo
