I went the other way. took off the 50t, put a 15t cog on the BSR, and bought a Schlumpf Mountain drive 2 speed crank. which came with 54t on a 130bcd spider.

In Low Range the crank turns faster than the chainring, acting as if a 21.6t

hub high, in crank low range, is the next lower gear. 6 ratios, all individual, no overlap. ~17" low gear .. so almost 1:1 with the little 16" wheels






...