I thought Nismo had adjustable cam gears, but their site seems to have disappeared. Maybe AFRacer can pick up some from Pitroad.

This is a classic technique; even the auto manufacturers retard or advance the same cam from year to year to shift the torque curve or reduce emissions. There is a great picture at stylinconcepts.com, but it appears they don't have these for a-series engines. Simpler ones have several different key slots on an otherwise standard-type gear.