For the bottom end you can probably a factory re-man crank for $4-500 with your core. This gets you all new bearings and con rods, just like new. These cylinders are nikasil plated, so if you bore them oversize you have to get them replated. TRhe plating is very hard, so you must evaluate whether the down cylinder is in need of replacement. If the plating is gouged through to the aluminum then you should be able to salvage the cylinder and just resurface it. You always want to resurface, fresh crosshatch, the cylinder when you put new rings in the bore. New pistons and rings are a simple and easy thing on these motors. Piston kits should probaly cost you less than $100 each. If you need a replacement cylinder a re-plated jug should be around $200 with core exchange. The base gaskets and head o-rings should be reuseable, but if you want a complete gasket set that might be another $50, or so. That is only guesses at the costs for the parts. If you pay someone to do the work I don't know what that would run, maybe $3 - 600. :dunno:
I'm going through something similar myself. In the spring a con rod bearing desintegrated and made a big mess at 8000 RPM. My current plan of attack is to use a brand new 550 sourced from salvage and put my top end on it. The 550 and 580 have the same bottom end. Swap in the stator and all should be good.