Cougar belt slip

[RichH]

Hi
I'm currently resurecting my vintage Schumacher cougar for me and the boys to have a go with. I've put a new battery in but it sounds like the belt is slipping as there's a tearing noise when you accelerate. There's no obvious wear to the pulleys and I've fully tightened the diff and belt but it still makes the same sound. What's the likely culprit? With spares being difficult to get hold of I don't want to waste time or money on parts that won't work. If it's going to take much to fix it I'm thinking of selling it for spares and getting something its easier to get parts for.

Thanks
Rich