Have you checked to see if your muffler barring is slipping? If its ok, just flush the blinker fluid reservoir and replace the headlight diaphragm.

 

Back in the day (305 scrambler Days) when the shifter got worn it would come loose to fix it we had 2 choice's replace or redesign the old one. We would remover the shifter place it in a vise to pinch it together than using a hacksaw through the pinch point put it back on and it then would tighten up on the shaft.
Damn that was along time ago! $ for Parts or $ for gas you guess where the $ went

 

Yep ... Been there and done that "Playx3" ... Ah yes, the good old days!

 

Lots of good suggestions, especially making sure the slot is wide enough to get the full pinch on the shaft and the hex bolt replacement. Another trick is to take the lever off and drill and tap for a 1/4-28 set screw to bite into the splined shaft after tightening everything else first. I have also drilled with the tap drill slightly (drill point depth only) into the shaft before removing the lever, but you still have to remove the lever to be sure the threads are all the way through.

 

Also check the length of the shank on the bolt. I've found that 9 out of 10 times it is just a bit too long causing the shank to bottom out before the lever is actually tight enough.
I replace them with bolts that have no shank.