Easy to side step. Call it a 2009 model Teddles490 on your entry form and you're home and hosed.
I am a scrutineer and I would not pass it.
18.7.12.1 Bikes will be OEM
Yamaha is the Original Equipment Manufacturer, not Nathan or Teddles.
Yamaha manufactured all of the Major Components of the bike in 1981.
Now back to the original question.
Evo is not a Period. That is why you can enter a bike in both Pre 1985 and Evo at a Championship meeting.
The Period of the 1981 bike and the 1982 seat and tank are Pre 1985 and so they conform to rule;
18.7.12.3 All components will be of the period the machine was manufactured
Therefore it is ok to use them.