What's your polo gear?

Go trials and fit your freewheel at the crank.
21:12 => 42.87
22:13 => 41.46
23:13 => 43.34

Or get a 35mm/1.35 rear tyre again so 29:17 is 42.2gi

Doing this would require changing most of my bike, or I could get a 16t like I said, rather than new cranks, new frame to take an ISIS BB and trials cranks, new ISIS BB and trials cranks and a new hub for a 12t sprocket.