The 4630 could have had a Delco starter which is manufactured by general motors. It also could have a john deere starter which was actually manufactured by nippondenso. Both starters are definitely rebuildable and yes you can probably find a solenoid for the delco at a GM dealer or auto parts store. But I agree with the others that it sounds like you have a bad connection somewhere. One area alot of people overlook is the cables themselves. If there are any repair ends at the battery posts, I would remove the repair end and clean the copper wire and the repair end to make good contact. Better yet, I would recommend replacing any cable with a repair end on it. These big diesels require alot of amps to crank and these repair ends lose too much voltage when they get corroded. If you get new cables, be sure to get heavy gauge wire. the originals are 00 gauge. Another item you dont want to overlook is the starter relay. It is located on the firewall under the hood, about 18 inches above the starter. One last thing, if you do need to remove the starter, the top bolt closest to the block is a real pain if you don't have a special wrench from John Deere, almost impossible. The dealer should be able to get you one, or if you know them well enough, maybe they would loan you one. Good luck!