>>1809150there is some overlap between single speed and IGH bikes, but they are not quite the same
look for the IGH first then find a single speed bike with the same or near enough overlock nut distance
.eg 118 mm or 108 mm internal geared hub
to go with a bike that has 120 mm or 110 mm hub spaceing
chain line may be a problem, given that it is the result of so many diffrent mesurements (chainline can be anywhere between 38 and 45 mm)
for chain tension some of this will depend on the type of Fork end on the frame
the shorter types do not allow much adjustment, so require more careful selection of gears
.eg a short drop out can adjust between 38 x 18 and 26 x 28 without changeing the chain length
external chain tensioners are mostly tension only in one direction so may be unsuited to a fixed gear or a kick back hub brake
bi directional chain tensioners are somewhat less common
