A jackshaft is basically an adapter. You run your 420 chain from the cent. clutch to a smaller sprocket, also 420. You simply take a piece of bar stock, or pipe maybe strong enough and much lighter, and weld a 428 sprocket to the other side, with the same diameter, and run the 428 chain back to your rear sprocker. Pillow blocks are a chunk of usually cast iron, kind of horseshoe shaped, that let you bolt them down, so the bearing can't move. It both locates and lets a shaft spin. The bearing is held inside the pillow block, you can get them in most any size.
The only thing about a jackshaft on a YZ80 is, where would you put it? how about this, are these steel sprockets on the rear, or aluminum? If they were steel, what about just getting a 420 sprocket welded to the 428 sprocket? Might not be pretty, but it would be simple. drill or cut out notches for the rear sprocket bolts, so you can take the whole thing off easy, and there ya go. I would think that would be the easiest solution, and probably the cheapest. You don't want too much in between teh front and rear sprocket, because you will have chain bind during your suspension travel. If not, email me a pic, I'll see If I can conjure something up for you, I MacGyver stuff for a living! Crexrun