80/81 has the looks , but nothing in the broeks (pants). I suppose what you do depends on what you want out the car.......if you want a showcar with no guts , leave it as is and have it serviced and checked over by a Corvette specialist.
If you want more "real" performance , you gonna have to spend money. For "cheap" seat of the pants performance , shortening the diff ratio to a 3.55 or so will wake the car up as much as you can without touching the engine. dunno if your box is auto , but if it is , a higher stall torque convertor will help too.
Engine wise , remember an engine is an air pump , so if you can get more air to flow , you can burn more fuel and make more power.
Key to this is a decent set of headers and heads and cam , in all probability your carb/tpi and intake will be "adequate" if in good condition.
My take on the whole story is that an 80/81 is hardly likely to become a collectors item , so I went the whole hog to get performance outa my 80. Being beaten by 2 chicks in a Golf GTI and them giving me the finger was the trigger. I didnt worry about keeping things original , but my car , unlike yours was tatty to begin with.
Perhaps your best option is to take out the original motor and stash it , and drop in a nice potent crate motor so you can keep everything original and still get some performance...I dunno......