Thanks IV73CI for the reply.
I would like to focus more on the head for now, and ignore both the bottom end and everything that will shift the powerband.

So, can you tell me why retainers and stems would need changing (ie, the theory behind it)?
And why would the oil pump need swapping?

Keeping in mind this is not to rev to 10,000rpm or anything, but from say an engine which can rev to 8,000, to about 9,000rpm

Thanks