What to do with a detonated top end

[FreeRide]

Hi Folks,
Really enjoying the forum. Ive torn my friends 02 gz250 (19k miles) down to find the source of a horrible sound, and after finding most everything fine, i pulled the head.

An exhaust valve seat chipped off, so the piston has some dents in it, and the bottom of the head as well. These all seem repairable, as does the bore, but my question is what to do with the head.

Can new valve seats be found and installed? On ronayers i can get whats called the valve spring seat (12933-38200), is this the broken part that is seen in the photo, to the right of the upper valve? It almost seems like this is a cylinder pressed into the head. If i can find the parts, i think i can do the work, or find somebody id like to give money to regrind the seats and valves, if thats something doable on these puny things. The valves look fine (surprisingly) but perhaps i should replace them altogether?

Or do i just buy one of these gz250 top ends on ebay for $360?

Anybody have good leads on finding these parts? Id like to keep the head running, seeing as everything but one seat seems fine, but i just havnt had any luck finding parts? Thanks guys!