Dan, first clean the pulley, the sub platter and the belt with 50% isopropyl and 50% "unscaled" water even if they look clean. You'll be surprise by the color of your cloth afterward.
Then just buy yourself a new belt. If not for now use, still to trial and sort your belt slippage at 45 rpm issue, then for later. Thakker's belt are good just like the Thorens ones at a fraction of the price.