Wow, if that's a legitimate 23 and not a replica, that's a hell of a car with quite a lot of racing pedigree. Personally I'd be almost afraid to drive it due to its rarity, but I wouldn't be able to resist giving it a good run at my home track. I've always had a love for Lotus, but somewhat at a distance until they started using Toyota engines in the late 90s. Maybe more than any other car company they embody the understanding that power to weight ratio has two variables, as exemplified by the famous quote from their founder "Simplify, then add lightness", and I've always really loved that. It's very much in line with my own thinking about cars and why the lighter a car is usually the more I love the feel of driving it. The Lotus Exige S makes my list because it has every bit of that design pedigree but the reliability of a Toyota drive train :). I'm not sure I'd want to actually try to campaign any of the non-Toyota powered Lotus cars, heh.
What's it like around the track in a 23? 900 pounds is already incredibly light. Unfortunately it would be impossible to meet modern safety standards while following the same design, but if you applied modern materials science to the car I wonder how much weight you could really save. Even in the 60s on the Lotus 23 they were using magnesium alloy drivetrain components and wheels, space frame chassis with lightweight skinning for body panels, and other techniques we still rely on. I would guess you wouldn't cut that much weight using carbon fiber body panels, maybe another 40 pounds or so at maximum perhaps? You'd probably pick up more speed by modifying it to accept wider tires and increasing the track width slightly, like some of the later replicas.
I think you and I would have fun in a garage wrenching over a few beers, glad to see there's others out there that appreciate the joys of lightweight beaters :)