That dark greasy junk is pretty normal on a motor that sat. Most of it is exhaust residue. Burnt oil works its way down the exhaust housing and collects right around the pump... they mixed the fuel a lot richer back then, so there is plenty of it to go around.
Way I tell them apart is the smell. Gear lube is sticky, dark, and it stinks of sulfur. Dry and sooty means exhaust. Was the gearcase low when you drained it? Did the lube look milky at all?
I would do the seals while you are in there. Cheap insurance on a 70 year old motor and you will not get a better chance. There are two on the driveshaft. The upper one is the one that lets lube up into the pump cavity when it goes. The lower one is the one that lets water in and ruins the gears.
Run a fingernail across the driveshaft where the seal rides. If you feel a groove, a new seal will not hold. That means a repair sleeve or another shaft.
Check the water tube grommet too. They go hard and crack, and then you are back in here next summer chasing an overheating problem.
Old seals come out with a small pick. Drive the new ones in with a socket that sits on the outer edge only. Keep it square. Grease the lip before it goes on the shaft.
Before it hits the water I would pressure and vacuum test the gearcase. Little ones like that do not take much, a few psi. Too much will blow your new seals right back out. Check the manual for the number... I do not know it off the top of my head.
That broken bolt, soak it a day in penetrating oil before you touch it.
Keep us posted on what you find. Thanks.
