Did the mechanic take the engine apart, or did he just tell you what he thought.
If your car won't start after it was overheating and you have a leaky radiator My bet is that you've warped the head or block as well.

Try this.
Take out your spark plugs and wires (make sure you mark which ones go where on your dist. cap).
have someone try to crank the car and while doing so put a hand over each spark plug opening. If you feel air push out then you have compression.. if you do not then your T-belt could be gone, or you could have a problem w/ the head or head gasket.