First things first. Need to verify that the transmission linkage, on the side of the transmission, is CLEAN. Seen more than one get dirt in/around it and hold the links from going to full forward. Once that is clean and working properly, then check your pedal linkages. If there is ANY slack in the joints, it will affect your travel speeds. If that is all in good condition, look on the transmission. There is a large hex, about 27mm IIRC, with a nut in the center. This is the transmission swash plate linkage adjustment. Sometimes one can adjust this and get a better top speed; but be warned that this adjustment is a last resort as it will also affect reverse speed and sometimes neutral creep.