The rear freewheel on a front freewheel system was a special Shimano friction freewheel. It had fiber spacers that allowed some freewheeling as a safety measure in case a vital body part go stuck under the moving chain. To my old memory, I do not remember anything that was special or different about the rear hub, so my guess is that you could remove and replace it with a normal ratcheting freewheel if you're removing the entire FF system. Do not use the FF System "rear freewheel" on a normal derailer system. It will not freely freewheel, and when the rider stops pedaling it would suck the rear derailer into the spokes.
I believe the bike was also equipped with the Shimano Positron Shifting System which worked together with the FF System. It may be different and weird, but it was a good working and bullet proof system specifically designed for kids, and adults that were intimidated by a normal derailer system. It was popular on the Schwinn Suburban models.
John