What happened to your engine then?
I mean rings I can understand. Valve seats never seen weared out byond repair. Yes okay, when they are destroyed by debris. But then you should need new pistons also I guess?
Sleeve seals can be bought plenty and have the same dimensions as from the common 845 engine.
I suggest if your pistons aren't worn out. You could suffice with a slight sleeve hone to take the glaze off the wall for better lubrication. Reseat the valves at the valveseats. And replace the piston rings.
New gaskets and it is almost new!