What diameter wheels are currently in the frame? Most SS/FG wheels you'll find will be 700C (622mm bead seat diameter); if that's what you bike currently runs it ought to be trivial to swap them in. If they're 27" (630mm bead seat diameter) you may need to get longer reach brake calipers to run a 700C wheel.
The low-budget option would be to simply install a single-cog freewheel on your current wheel (assuming English-spec hub thread).