On the output side, the coil primary should be on the drain side of the NFet. An ~60V Transorb snubber between the NFet drain and ground would protect the NFet. Depending on the FET, its input capacitance may be too high for the uP output pin to drive it. If you want to use the uP pin to drive the gate directly, the NFet must be a "logic level gate" type. The coil is probably an autotransformer, so it only has three terminals. You can only drive one spark plug per coil, or you must use a HV distributor between the HV terminal on the coil and multiple spark plugs.