So you need a 6-speed (or narrow 7) screw-on multiple freewheel and some kind (pretty much any kind) of rear dérailleur with a more or less matching lever. I'd stick to 1×6 for simplicity's sake.
@gbj_tester started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
So you need a 6-speed (or narrow 7) screw-on multiple freewheel and some kind (pretty much any kind) of rear dérailleur with a more or less matching lever. I'd stick to 1×6 for simplicity's sake.