[Gav]

Hey Scoob, don't panic we are all freinds here even though it may not seem it from time to time.

One of the CR guys has used MX5 seats with some success, this may be what you need.
I have used SAAS seats which are a discontinued model but even these where not really high enough to properly support your neck.

Have you considered a separate headrest mounted to the roll bar? They could then be removed if you are after an 'original' look.