Options really are:

Standard (Not an option!)
Vauxhall Calbira Turbo (Good for 300bhp)
RS200 Pistons (To acheive a low enough CR you have to take a lot of meat off so the crown gets very thin, not a good idea)
Acrillite (No experience of these)
Then proper places like JE, Arrias is it?

Calibra Turbo are the best VFM PhilM does a set for around £150 on here or new they are about £400 I think.