Reducing the pitch is ok to a certain point. You want the motor to run at it's given max RPM's when it's at wide open throttle according to the manufacturers specs. OK now remember that little motor may have to take you in some day if you main engine gives out. It has happened to us in the past. So MFG specs. are important unless you want go back home at a real slow speed. Now with that given if you have a 2 blade prop see if you can get a 3 blade for it. If it's a 3 blade prop see if you can get a 4 blade for it. Reason behind this is you're within the given prop size the manufacturer states you need for your motor the extra blade will slow RPM's a touch but give you more torque too which is what is really needed for a little kicker trying to push a large boat through the water. That's why all commercial heavy use boats have 4 blade props. For the power and torque , not speed. Start by going to the MFG web site of your motor or call them directly , find out the original prop size for your motor and take it from there. I know you're trolling with it but by running to small a prop when you need to use it get home and you decide to run her wide open is a disaster waiting to happen. Over speeding the motor will trash it fast. Like I said , easiest solution is either a 3 or 4 blade prop and that depends on if the original was a 2 or 3 blade prop also.