not all expresses have the same engine. there were 4 (excluding the SR and the express City)
1-speed spring start w/points (NC50 1977-80, NA50 1979-80)
1-speed kick-start CDI (NC50 1981)
2-speed kick-start CDI (NC50 1982-83, NA50 1981)
variated CDI (NU50 1982-83)
are you sure it has cdi and spring starter? i dont think that combination is possible, unless somebody trasnplanted a CDI unit onto a spring start engine.
if yours has a spring starter, it would be 1979-80, but if it has cdi, it would have to be 81, and it wouldnt have a spring start.
the only way to put a variator on an express II is to get an urban express engine and do a complete engine swap. the cases arent the same.
before i discovered puchs i was HEAVILY into expresses.