I drilled through the top portion of the gear shift-lever clamp and through the gear shift rod, then tapped it with 1/4" thread. With plenty of lock-tite I fitted a stainless bolt through the lever and into the rod until it was nice and snug. Now as long as nothing cracks while the bike is running there will be no more slipping shift lever. I'll keep my fingers crossed on this one. Next step would be to replace the whole shifting mechanism which requires getting into the inner timing cover and a fairly expensive replacement shifting mechanism.