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.
Look on kbb.com for the value. You will have to look in the collectors' section. Is the car a Shelby Cobra? If it is, it will have an additional value. In any case, you should be able to get about $1500.00 for it if it runs well. If the motor is in need, expect about $350 to $400 at best.