I didn't see/read your other thread.....
I'm gonna go out on a limb here and guess you don't store fuel for a couple months before you put it in your machine, Or let your machine sit a couple months between each use right ?
If not.....I'm inclined to call BS
on what this "dealer" is trying to pass off on you.
Now, This sounds more like what has happened;
They said that bad gas had created carbon build-up on one of the valves creating blow-by that has ruined some of the gaskets but not the engine.
Blow-by/bad valve guides or seals create carbon build-up on the valve/piston and head not bad fuel, Strange that I don't recall finding "ruined gaskets" on any engine I've tore down with carbon issues.
Being a Kawi warranty Tech and not able to inspect this engine up close and personal and going by your post alone, Something sounds fishy.....
IMO, A second opinion is in order.....