I have a 2007 AC 90cc ATV. Engine is bad. I assume that AC used same engine across multiple years? What year(s) engine besides another 2007 would bolt into the 2007? My 2007 engine ESN is AC20 *1401523*

I can't answer that but can open up more options for you. Kymco makes the Arctic Cat 90. They also make the Kawasaki 90. So you could look around for those three close to your year model. The biggest difference I know of is including reverse and a bigger gear box as far as oil capacity on the later models.