another source of a chain tensioner is any short cage you can lay your hands on,
center the pulley over the cog and keep it there by turning the limit screws in
Until they meet..

clamp a short bit of cable in , if you wish. housing shoulder to the fixing nut.
it doesnt have to hook up to anything else.

Sturmey archer drum brakes work fine ,
my set uses a derailleur and a 6 speedfreewheel on the rear

sram makes a Spectro S7 drum IG 7 speed

sturmey's drum brake 8 speed hub needs a rather small chainring
As Its 1:1 ratio is the low one , all the others are overdrive gears.