Hi Mark,
Welcome to the Corvette Action Center. The original engine had the car's serial number on the flat surface in front of the right head along with a production code. Other engine accessories were coded too and would have to be correct if you are looking to restore the car for NCRS judging. If you just want to build a nice driver or non-original show car you can use any small block Chevy or even a big block. A new crate motor would be a good choice also.
Personally I'd ask questions and try to locate the original engine and trans even if I didn't want to use it now. Having it would increase the value of the car should you ever want to sell it.
As for value that depends on the condition. If the body is really nice and the frame isn't rusted it should be in the mid teens. More if it was an original fuelie; less if just a base motor car. Look in Hemmings, Corvette and Chevy Trader and others to see what similar cars ar being sold for. Then try to figure out how much you will invest in the car to get it where you want it. It will always take much more than you figure.
Got any pictures?
Tom