When you take on a Corvette restoration project, one of the major projects (but not always the biggest project) is the restoration of the engine. This is not only a matter of your Corvette's power and performance - with older 'Vettes the engine is a critical piece of the car's history. Originality is absolutely critical, but often difficult to achieve.
The choice before you when you consider your engine can be complex, even before you consider how much it's going to cost. The first big decision is whether you should rebuild the engine that's in the car, or go out and buy a new one. Here are the factors to consider:
The good news is that most small-block Corvettes were built with Chevy's 4-bolt main 350 cubic inch engine. These engines are readily available - you can even buy a brand new "Goodwrench 350" engine at any Chevy dealer. The Goodwrench 350 costs less than $2,000, and is rated at 195 net horsepower, but that quickly rises to 260 or better with the addition of a free-flowing exhaust and an aftermarket intake and carburetor. Avoid used engines - you don't know what's inside them, and neither does the guy selling you the engine.
With newer models (such as the LS series), you can buy those engines new from your Chevy dealer, but they'll cost more than the venerable 350. Obviously, less common engines are harder to find and not made new any more, so you're looking at a remanufactured engine for your 283, 327, 396, 427, or 454.
Visit the GM Performance Parts site for a complete list of factory-new engines for your Corvette.