Surely there's nothing wrong with the equipment, you've just missed something on re-assembly. If everything worked prior to head coming off then the metering head is fine-they don't just stop working for no reason (never seen one failed yet). Check all timing, all wiring, pull the flow off the m/h and check you have fuel, then either pull an injector or remove a plug to confirm fuel getting in. Check things like throttle microswitch for correct function, and all vac hoses, electrical connections etc. I assume your pump primes fine etc on ignition? I've had my kr 9a and abf engines in + out a million times, and its never a hardware issue if you havve starting/running problems, its always been because I've left something off and either forgotten or over looked it.