250-500cc Wr300 2009, Drinking Coolant- Went Bang :/

Afternoon,

New owner / new member.

Purchased a wr300 2009 earlier this year.
Bike was previously rebuilt in November of '20 by a local shop.
First ride bike drank coolant and started steaming (out the exhaust) on decel.
Took back to shop and they said it had blown an Oring in the head.
They replaced it ad charged me labour only ($360).
I replaced rads, all coolant hoses (removed tStat), fresh coolant and checked waterpump operation. All fine.

This weekend just gone i jumped on the bike (Coolant full at this stage), got not even 10 mins up the road and it sized at some speed north of 80km/h. It then let go and there is no compression.
Also very low coolant.

I've called the shop today and i'm taking it back to them, but has anyone had these issues before?

Has anyone had to replace the bore before? As I assume the ring has grabbed the exhaust port (or similar) and done some damage.
Or do a rebuild after a major issue like this?

All help appreciated.

Matt