Check the fluid. It should be thin and clear with a reddish tint, and have an oily smell. If it is dark and smells burnt, there's your answer. If it's not too far gone, a fluid change (maybe the filter too) might keep it going a while longer.

If the car is anywhere near intact, not rusted to death, it is worth preserving---there are so few 140s of any kind left nowadays. If it comes to swapping transmissions, think about a manual conversion. It makes the car a lot nicer to drive, especially if you get one with overdrive. It's a lot more work, (changing pedal assembly etc) but well worth it.