I need one for a 20A1 chassis. I found a 7500 50W from Mouser that's almost the same size as the original.
http://www.mouser.com/Search/Product...ey588-L50J7K5E
Nuts, I see you need an 8500 for the 20B1 chassis. I checked Mouse, Digi-key, Surplus Sales of Nebraska and Allied. Nobody has 8500
Maybe you can string together a couple in series ?