strynboen, my ss is 50cc with 16mm carb (a PO had 19mm installed which was way too big so i replaced it).
here in the netherlands they were de-tuned to go 45kph only which is the legal limit for a moped. this was done with a spark "retarding" mechanism in the flywheel, like what you describe. i replaced it with an advancing one.
with that, it can run up to 80kph now but you need to be really patient to get there. i think the ss head has larger valvles than the cd. (ss - super sport - ha ha!)
like you, i would officially need to register it as a motorbike to be legal but would then lose some previleges in the city, like being allowed to go in one-way lanes against direction of traffic. so i just risk it and hope to be able to play the innocent "such a cute little old thing" card if needed
the amigo is cool. you're planning to restore it?