I personally would never tell anybody to go out and make a new data plate. No sir, not me. Never. It is probably a crime - and they have ways of getting you to admit it - a rag and a pitcher of water?
Sounds like you already have the paperwork for the 85.
If you cannot get a field approval for the seats and tanks, convert it back. Or look for an STC, and put the correct tanks in there.
If I didn't already have a Cub, I'd be looking for a 90 hp. L2 with spoilers! Great fun!