If they've informed you of the risks, then rectification would in all logic be at your cost, should they snap any.
Fixing it ... down to competency within that business as to how they approach this.
Most assured outcome would likey involve the head off and to a machine shop to mount on a milling machine, then to machine the broken part out ( especially if stuck with nothing protruding) to not balls up the head casting.
A friend, when these were contemporary engines, ran a jig on milling machine to do this and to machine the spark plug holes, especially two centre one's, in repairing them. All his customers were bike shops that farmed this out to him.
Difficult decision to go ahead or not, they can easily go wrong and they are right to warn you of this.
May be lucky if they've originally been put in with coppaslip or similar.