A screwdriver! Tis all I needed, and had, thank gawd. if you have AN fittings, then the approproate wrenches or a West Virginia Socket Set (Crescent wrench). I've always carried in my cars, and now the boats, a tube of RTV Blue and Black. Never needed them, but always thought I might. Small tool set in a case is not a bad idea. STD and not Metric would be nice. Nordie even mounted a tool box in his, right in front of the engine.
As far as parts, no matter what part you bring, it won't be the one that breaks. Murphy's, and Ray's, Law. But you might be luckier than me. Since the advent of the boats, I've carrried a lot more in the truck. Jumper cables (In the boat), battery charger, extension cords, garden hose, and a Unilite Dizzy (in case Skip actually doesn't Stiff out on us).
J/K Skip, but gotta raz ya on that. I'll carry that dizzy in the truck until I get a new truck!
Ray