I took a look at your picture of the transom and I can just make out the hull ID number (AMF46238M73A) which would make it a 1973 hull if I read it correctly.
I'm pretty sure that Michigan DOES require you to register your Sunfish (no title, though). It's $9 for 3 years. Just have the previous owner write down his info, your info, along with the hull ID number and the selling price.
title is only required if over 20-ft in length, or motorized.All watercraft on Michigan waters, including privately owned lakes and waterways, must be registered except:
Watercraft 16 feet or less, propelled by oars or paddles, and not used for rental or commercial purposes;
Non-motorized canoes and kayaks not used for rental or commercial purposes, rafts, surfboards, sailboards, and swim floats, regardless of length;
Watercraft registered in another state and used only temporarily in Michigan.