Did you examine the cam bearing journals for any embedded aluminum--and on the head the cam saddles for gouging? The cams can and do sieze up for lack of oil (delivery passage along one head bolt crudes up--or the passages in the head itself). If that's the case you can address it without removing the head (wire and solvent and a couple of oil/filter changes). The cam bearing surfaces can be cleaned up with emery cloth--the head saddles too. Is the auxilary shaft spinning freely. The idler pulley shouldn't stop the belt from turning but have you tested/replaced it. Let the idler pulley set the tension--after installing the belt advance the motor a few degrees from the crank pulley to put the slack on the idler pulley side then tighten the lock nut. The bits of rubber under the lifters is what remains of the "cushion". They usually get rock hard with age/mileage and then fall out of place. -- Dave