I have an old Roper tractor mower , Model No GC220BR, LOT NO 0478M, PART No 103?9?77X and the drive belt (I think? - the belt that turns the blades!) has come off / shredded / needs replacement. How can I find out the right size? My local service provider said it was too old for him to find spare parts for but I reckon I might track something down online or could maybe use another brand if I knew how to measure it. I am in the U.K. ; don't have a manual; haven't managed to find a UK supplier.