Bill is correct. You should ask MaxProp to suggest a size and pitch settings. I think it could be different than your existing fixed prop due to the MaxProp blade design and size.
FWIW, the prop size should be stamped into the hub on your fixed prop. A picture of my original two-blade prop for my h40.5 is should below for reference. It shows the manufacturer's name, RH=Right Hand rotation for forward drive, 18" diameter, 16" pitch, and sized for an 1.25" shaft.