6 hp B&S Diamond vert shaft

I am not the sharpest tool in the box with small engines, but I have taken a 6 hp B&S Diamond engine from a pressure washer and want to install it in my old JD 100 round fender lawn tractor.  Not a lot of power, but it is only for moving the tractor with occasional trailer, not for attachments.
 Question:
 The engine, being on a shot (Broken pump) pressure washer, it is obviously set to run at a constant speed.  What must I do to set it up for a regular tractor varying throttle use?

                                                                                                                 Thanks, ...Ironart