Johnwa I have used jb weld on external engine repairs before but I am somewhat concerned being oil soakred with lots of vibration and heat that a failure may occur. If it gets caught up in the chain and the engine being an interference engine bad things may happen.
Interesting I believe those rods are simiar to durafix low temp aluminum brazing rods that require mapp gas. I have another head which is awaiting recycling I could drill a hole and you could see if it would work out. If you are game emai me and we can discuss. We would just need a straight edge to measure the matting surface before and afterward. Oh and have a copper pin made.