Hi Kevstar,
I too just purchased an '81 KDX 420 for a complete resto. They are relatively rare, only made for one year. It became a 450 in '82 and then was discontinued. Most parts are available, but unfortunately not the piston rings. The piston is, however, as well as gaskets, etc. I use bikebandit for my Kawasaki parts supplier, VRM Racing for seat covers and tank decals, DC Plastics for plastic (overall dull matte finish but good quality), and ebay for everything else. Keep looking on ebay for parts, it taskes persistence and sometimes a general search- for example if your tank is stained brown, look under "kawasaki tank" or something like that for mislabeled stuff- I did that and found a really nice tank for $2! If your other plastic is chalky white or faded dark green, as it tends to, you can restore it to good condition by scraping with a razor blade down to good plastic followed by sanding up the 600 grit.
I'd definitely take off the top end- the piston to bore clearance was really tight, 0.002" on those, and ring end gap should be under 0.030". If the crank big end doesn't have any noticeable sideways twist or end play, it'll probably be fine.
The suspension can actually be made to work pretty well, if you send the rear to Works Performance and play around with fork oil weight and levels in the front.
Most people are surprised at how well a properly set up, decent quality post-vintage bike works, but it all depends on how much you want to spend. Honestly, they are super easy to work on, if you need a manual I can email you scanned images of specific sections, but being old bikes they really need a compete going through to be decent.
Sorry to ramble a bit, but my passion is post-vintage dirtbikes- with the exception of the '85 M-Star 500 I just bought to restore- disc brake (front) and water cooling- now that is getting pretty modern!
Let me know if you have any questions, I'd be glad to help if I can- love to see those old beasts flying through the woods.
Mike K.
