I found a Bridgestone 400 frame, fork, crank brakes and seatpost, in the dump abour 15 years ago> built it up with 700c wheels, 32mm wide Paselas, and some other parts I happened to have. It was, and still is, a nice bike that was my favorite bike, utility bike, light tourer, etc. for a few years.
I would not put 26" wheels and a coaster brake on it. 650B might be possible. I think it would take 42mm tires. I hope whoever converted yours toa a fixie didn't butcher it by cutting off the rear derailleur hanger.