The Pro Compe 5 speed body has 2 stages. The inner 3 have 4 splines (slots) and the cogs simply slide on/off just like a cassette. The outer 2 (smallest) are threaded, so you have to restrain one of the inner cogs while you unscrew each of the 2 outer cogs. The easiest way to do this is to use a bench vise and a cog holder tool to hold the freewheel by one of its inner sliding cogs and use a chainwhip or track cog tool to unscrew the outer threaded cogs. Needless to say, you remove the outmost cog first before removing the second cog. Since your 20T is in 3rd position it will be splined and any Winner, New Winner, Winner Pro inner 20T cog will fit.