Just get a Shimano external-BB compact chainset, take off the big ring and fit either a bashguard or single-ring chainring bolts. Chainline should be close enough.
With Kojak 35-622 tyres, a 34:22 gear ratio should give you an on-trend 42" gear ratio.
That's the same gearing I used when riding my old pomp for polo. I managed to get a 16t on the other side to give a reasonable gear to get to the courts on.
Just get a Shimano external-BB compact chainset, take off the big ring and fit either a bashguard or single-ring chainring bolts. Chainline should be close enough.
With Kojak 35-622 tyres, a 34:22 gear ratio should give you an on-trend 42" gear ratio.