[Dominik]

I would buy one from a dealer you can trust (...) or a workshop that you know.

For specifications (In the last 30 years I drove about 50 Cobra Replicas, one original, and owned a dozen): A mild cam, big cube motor (as mentioned before) with rear exhaust is what I consider next. And moving to the mountains outside La Canada to have fun with it.

A 680 hp, lumpy idle 496 cui was nice, but I found myself more often driving my lady's 911 automatic convertible than the Cobra.

Plan to work a bit on it yourself. You don't want downtime just because a water hose popped.
Or the headers came loose.