I owned a 1986 Mazda B2000 for 11 or so years. Eventually had to do a rebuild on the engine at 137000 miles. Unknowingly, my thermostat gauge didn't work and I overheated it at 148000 miles and cracked the head. Had a friend who found a 1988 2.2l that I dropped in and never had another problem with it. I did install an electric fan though! Sold it at 202,000 miles.
I changed the manual tran. fluid to synthetic (as well as the differential) and it shifted 100% better. Changed the clutch when I did the engine swap. Had to replace the differential at around 190,000. I beat the truck up alot though, so I'm not sure if I could give you an "on average" picture.
Those trucks were pretty solid, but overheating was always an issue with them. I had alot of problems with spark plug wires cracking due to their placement over the exhaust manifold. Carb needed to be rebuilt when I sold it. The butterfly wouldn't open/close correctly. Ball joints were shot as well.
If you want to know more about them, just ask.