John, I had a nice 3 speed motor that would have probably fit right in there for you. I tried for over a year to give it away for the cost of postage and finally somebody took it. Many 12v motors from hang-down units, aftermarket units, etc might fit. I'd suggest removing your motor, measuring everything and looking on the large internet auction site. I would suggest a step-down converter (12v to 6v) however the amperage might be the stumbling block. Those 6v motors took between 5 amps and 10 amps and if there was extra drag from old/deteriorated bushings might take over that. A 12v to 6v step-down with 15 amp ability and a proper heat sink will cost you the same $30 a Chinese made 12v fan motor might cost...and you'll still have an old motor.
