If it's just for fun, then find an old set of wheels from before cassette
hubs - one with a freewheel. Then remove the freewheel and screw on a track
cog.

i wouldn't do that - you can't get a lockring to stay on. the one time
you really need to backpedal effectively, it'll unscrew.

i know - i have a scar from when i encountered this issue as a kid.

jb