This should work. The typical 6 HP Briggs or Honda engine runs open throttle at 3600 rpm. You need a 6 to one gear reduction on the motor. Commonly available. Or use a v-belt drive and reduce it 6 to one; then you have a speed of about 600. Throttle back until you have a speed that suits you.
Is the sprayer on a trailer--or--do you plan to slide it onto your pickup? Do you drive on the lawn using the boom or use the hose? Or use a utility vehicle?