Disclaimer: all my knowledge pertains to Starion/Conquests with the turbo variant of that engine.

Yikes, HG's are a pretty common issue on the G54. Do you have a jet head or non-jet head? Anecdotally non-jet are less prone to cracking. Starquest guys seem to like this source for parts and expertise:
Engine Machine Service, Inc. - Conquest

Many of the parts swap over to non-turbo variants, probably couldn't hurt to give them a call and pick their brains. Never fun to deal with those issues, good luck with it Tom.

__________________
TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth