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On your MTX-3, a 22T pinion with a 42T spur will give you 4.82 final drive ratio. A 23T pinion will get you 4.61 drive ratio which What you could do is you could check if the 24T pulley conversion from the V1RR could be used on the MTX-3. It comes with the necessary belts. The 24T pulleys is meant to swap the 19T pulleys on the V1RR. This changes the internal drive ratios to 2. I don't know if they can fit in the MTX-3 which also uses 19T side and 19T brake pulleys. Then the options for gearing are finitely possible to reach with an internal drive ratio of 2.
Check out Kawahara. They have a new set of pulley came out wth lower internal drive ratio. Now only come in Kit form which incl other kawa stuff, but should be out in separate item# next week. Looking forward to try it out.