Ok, So this is a bit of a nutty idea.
I've just replaced my old wetstone grinder and it dawned on me that the 130 RPM drive to the 8" wet wheel might drive the gearbox directly. So, a new 5/8" drive shaft with bored out nylon bushings and gear wheel later plus a gash 20 tooth drive gear and second wheel completely amputated we have…

Plenty of power but not quite the nice 16:1 reduction of Martin's gearbox.
However it'll do as a starting point FOC. 
Jon