Jim, first thing to check its the ground- wire going to the starter area of the engine, from the battery. Be sure it is clean and is getting a good connection on the engine. If it is good I would follow the red wire from the positive+ side of the battery and in that box is a solenoid that operates the starter. From the wire coming in from the battery use a 14 guage insulated wire and jump across to the other large wire going to the starter. If the starter turns over the solenoid is bad if not it could be the starter. Let us know how you make out.
Karl