Anytime that a reduction gear change is contemplated, especially numerically larger such as this case, propeller shaft diameter must increase and also propeller size as well as strut bore sizing. As to propeller size, presuming that the diameter is at its maximum sizing considering the blade tip clearance to the bottom of the boat, the only other option would be to increase pitch depending if it is allowable as to the prop's make and model. For example, the Michigan DynaQuad only allows an increase of pitch over diameter by 10%. In other words, 20"D x 22"P. Check the manufacturer's prop specs for accuracy as to this aspect. Some of the more recent props on the market by Michigan, Veem, ZF, and Acme are more tolerant to this ratio. Our boat has 24"D x 37"P x 4 Acmes as an example which is quite radical from the older wheels.