if you are a digger and willing to experiment, you could disassemble the rear hub and see how the brake works and possibly remove parts to taste. i'm pretty sure i have a recently manufactured sturmey archer coaster brake hub that if i removed the brake shoes on (very doable) would most likely freewheel. barring that, determine the width of the rear dropouts and try to find a shimano compatible hyperglide rear hub with the correct spoke count. even an old style threadon freewheel type hub (more easily found in narrower rear hub widths than current designs) can work with a single speed freewheel. remember that your bike may be set up to only run with one sized rear cog, using the current front chainring, that is, and if so, you would have to make sure any new rear hub has a cog available in that number of teeth.

this is just a start. it could be real easy or almost impossible economically speaking that is. others can add more info, confusing the issue even more than already have...