[Chambers]

Well I don't honestly know about that sound you are hearing, possibly could be a damaged valve, but did you at least change the oil and filter before you went and revved it out so much for so long?

Also how is the oil pressure when you put some rpms on it? Does it fluctuate?
Lastly, if you are serious about this car, and you want to keep it, and if you are willing to put the money down to get the NA back to stock, find your self a turbo engine/tranny/harness/ECU and swap it in. Because if it is a damaged valve, you will probably need a new head unless it has not been damaged, and for what that would probably cost you could get a nice turbo engine. As for the hg, they wernt 'weak' but the torque specs were no where near what they needed to be so the hg would blow, ARP head studs and some type of metal head gasket are ideal for a bhg. All I can say now is good luck.