My new to me 420 has 7,000 miles and 670 hours and was smoking bad. Tore it down, and all looked good inside except head needed bored over .30, new rings and piston. If you can hold a wrench, $110 for machine work and 150 for parts, will get these things rolling again. (I pulled motor as I had trans trouble as well, clutches, timing chain were still in great shape w 7 k miles!).