Built in rev limiter on a 420? Like the new Hondas? If so, get a non-rev limited 420 flywheel, compare it to your flywheel, and check and be sure the magnet is secured at the exact same location in relation to the keyway. if so, I would suspect you could simply bolt on another coil. If the flywheels are different, you will need a flywheel and coil from a non-rev limiting engine. How many wires (other than the spark plug wire) are coming from the coil you have now?