I have a 1976 mustang the motor needs work but runs and ?
It's only worth what someone will pay you for it.
It's almost impossible to determine a value sight unseen, but that year of Mustang isn't very popular since the later models had stronger engines, they looked better, and can be bought in the condition you describe for less than $1000 US.
Your best bet is to try to find someone that will trade for the car you would really rather have, like a used car lot. Engine problems really torpedo the value of used cars.