If the engine has been setting around for quite a while you probably should replace the gaskets and diaphragm in the carb. Your local small engine shop usually has the kits. Also Zenoah magneto engines need to be very wet to start, I always put my thumb over the choke and prop the engine until my thumb is wet. Usually starts in three or four flips. Make sure the prop is lined up with the hub correctly, on a two blade prop the blades are across the notches in the prop driver. Zenoahs like RPM A good alternative to replacing the magneto with ignition is a C&H jumpstart. They work really well and make starting easy without doing away with the magneto.
Larry