Originally Posted by
Sixty Fiver
SA still makes new cogs...
...and in fact S-A cogs for their 2-3-4-5&7 speed hubs are interchangeable with Shimano cogs for their 3-4-7&8 speed hubs which are interchangeable with SRAM (F&S) cogs for their 2-3-5&7 speed hubs which are interchangeable with the cogs for many single-speed coaster brake hubs. These cogs have been an industry standard for some 60 years. You pop the retaining ring off with a flat blade screwdriver to change 'em out.
...and dropout design on the Twenty really works better with an even / odd combination. The dropout is short and is angled upwards so there is not a lot of room to play with chain tension... a 17 or 19 would be an easier switch.
If an 18 will give you your most optimum gearing, use a half link.
HTH,
tcs