Novus plastic renew. You are never going to get plastic new-you'll have to buy that. Do a search on the net for it. I have a link for it at work. I use a bleach water concentrate and lightly brush it in with a soft bristle brush and let it soak. Come back with a heavy concentrated simple green, rinse good and let it dry competely, then, follow up again with a light diluted simple green wiping more, especially the frame, then...just use some tire shine product, any, and you be amazed at the difference.
Do this first though...Use some Gunk on the engine and let it sit for about a 1/2 hour when the motor/frame is cool. Come back to it, try and scrub the heavy soiled areas and powerwash off.....ah....everyone will want to know your regiment.
I just sold two atv's over the weekend. Both sold when the first people saw them. They couldn't believe the condition and how clean I had them too.