In order to put a fan onto a '48 generator you would need a '39 pulley, in lieu of trying to do this, you could just use a '39 generator.... Very hard item to find.. The '39 generators are 2 brush, higher amps, in lieu of 3 brush like the earlier generators.. The '39's came with a voltage regulator, like the '40 and later, in lieu of a "cut-out" like the earlier gen's.