[btom]

this sounds like the valve rockers were riding tight on shims at Top Dead Center and the front cylinder head got fried from excessive head, classic example of not checking valve shim clearances on all valves every 6,000 miles or 10,000 km. You might have to split the case and do a deep dive inspection on internals, dunno, I've not done that before but I'm a cautious type of person and if that were my motor I'd do a complete inner inspection first, to see if there's anything else going on

 

[btom]

there might be that used dropped valve shim still in there, or it might have ended up on top of the damaged piston

 

[btom]

Oh good you found that shim, and for the cylinder jug, there's no guarantee on the good condition of purchasing any used jugs, so I would be tempted to try to repair/reuse what you have, it couldn't do any harm

 

[btom]

I am lucky that when I bought my '05 800 that the previous owner kept maintenance records so I could see in the Owners Manual when he last did a valve shim clearance check, keeping maintenance records is necessary I think