what year? is it 12V? did it have an ignition resistor on it previously?
what you describe sounds like the power > ignition switch to coil is missing, you get coil power when the starter is engaged so engine starts, but that power is lost when you let off on the starter, and the wiring [that might have gone via an ignition resistor] for "normal" power to the coil is missing, engines dies
Bill
