You say, you only have one magnet. What is the code (name of your ignition module) Is it a black potted box, a film roll type, or a tin box ignition.
the old type vlach that used one single changed to two magnets, but this will not operate with the single magnet.
Engine power should not be ignition type dependant. Ask for a new ignition. The one they sent you clearly is not OK. Don't blame the engine if the fault is somewhere else.