Good: You know what freewheel cog size you need to match your chain.
Bad: You haven't told us exactly what kind of rear hub you have.

If you have a freewheel hub (which I suspect) you could remove your multi-speed freewheel, substitute a single speed freewheel of the appropriate size, space the axle and redish the wheel as needed and you'll be good-to-go.

If you have a cassette hub, take apart your cassette. Pick out the 1 cog you want to use and make spacers with some plastic pipe or whatever else is available.