A rule of thumb that I've found somewhat accurate , is that the speed increase in % will be half of the HP increase in %. <br />So say you increase HP 20% to ~110HP , your speed should then increase 10%, or by ~4mph.
Just this year I replaced a 85hp Suzuki with a 115 hp Evinrude on 17.5 bowrider which has a pad semi-V hull. Both engines were running good. The 85 hp pushed the boat at 40 mph GPS and the 115 pushed the boat at 49 mph GPS with a Stainless prop and a jackplate. 9mph improvement and much better hole shot. Average fuel consumption for a trip to my usual spot 6 gallons for the 85hp, 7 gallons for the 115hp. I might add that there was a 4 mph difference with 2 props I tested on the 115hp. Based on your 42 mph I would guess you could hope for 49-51 mph top speed with the best prop. Hope this helps.
