That is a well balanced power unit you have. By switching that prop you only picked up .5 of pitch. If not the same brand prop it is possible that both props will have the same thrust. You can try the same diameter but add at least an inch or inch plus .5 to the pitch. For more rpm you will have to go to a five pack but make sure the motor will handle it. Most of the name brand motors are rated conservatively and you can control the heat by propping down a bit. The Harmon Rocket was simply an RV homebuilt with larger engine and higher landing gear to handle the larger prop. It is a nice airplane but is rather draggy. Norm