1978 ,
but if it’s just a plain Maxi Model not Newport, maxi luxe, Sport MKII etc, it’s the same e50 engine as 80-81 model that has open port aluminum cylinder, 14mm bing and intake (if 2hp) Otherwise all maxis had High torque cylinders and only 12mm bing,
They dropped Maxi II, Newport L, maxi sport with e50, Red magnum XK, Silver magnum MKII But kept the plain Maxi model for 1980-81