+1) Leave it as a Freewheel 1 speed OR, Get a proper fixie Hub wheel
they are threaded twice.. Cog goes on RH thread , lock ring goes on LH thread.
OR.. [More, spending involved)
SRAM Torpedo Hub can be fixed or freewheels with a screwdriver turning a locking pin in or out..
https://www.sram.com/sram/urban/family/torpedo
Sellers, with a QBP account, Many.. this one of them
https://www.benscycle.com/p-3714-sra...Nm0RoC_lrw_wcB