Lippincotts were probably the best-made Lightnings and fast, too. My last boat was a glass Lippincott in the 9000's, one of the first glass boats. She had no tanks at all, the only flotation being the core in the hull and deck. I often wondered if she'd float at all if capsized. This model and the early glass Allens are referred to as "sinkers." She was a very fast boat, though.
A couple comments: Remember my earlier advice: before you do any structural work, get some experience cruising sheltered water. I promise you, you'll be glad you did.
As I recall, 11,000 series Lippincotts had significant flotation built in. Your boat might not need much, if anything, additional. No Lighting that I know of has a water-tight, much less air-tight, hull-deck joint. That joint is expected to never be immersed. The only time this causes a problem is if the seam is part of an air tank. Nickels Boat Co. solved this in their Lightnings by having all their flotation in the hull. I think Allens do now, too. As you can tell, it's been quite a while since I've been active in the class.
Remember that any flotation under the deck, while it will prevent the boat from sinking, will be no help at all in self-rescuing. Visualize the boat upright, but swamped, with the deck at the water's surface. What you need in that situation is flotation low down in the hull that will push the boat up out of the water. In order to rescue herself, the swamped boat needs to float high enough so that the slot in the top of the centerboard trunk is out of the water with the crew standing in the boat. This is why modern Lightnings all have double bottoms. You also need a way to get the water in the boat out. This is why boats have transom flaps and a couple of large self bailers. You can't bail her out with a bucket; waves will keep slopping into the boat as fast as you can bail. She must be sailed dry. As the boat gains headway the bow will rise and the water inside runs aft to exit through the transom doors and the bailers. Modern boats have such good flotation (and frankly, capsizes are so infrequent) that many have only one large bailer and no flaps.
As for the integrity of air tanks, keep in mind that a boat righted quickly can tolerate some leakage in the tanks. If you are up and underway three minutes after dumping (as you should be) a pint of water sloshing around in the bottom of a tank is of no consequence. It is only when the boat is in distress for a long period that the air tank's ultimate integrity becomes important. I recall a Thistle regatta on the west coast of the US many years ago in which a boat dumped and went unnoticed by the fleet and the race committee who returned to port after the race. The crew couldn't get the boat up and were out in the ocean overnight. When found the next day, only a couple feet of the bow were still above the surface. Only one crew member survived.
Gougeon Brothers used to have a manual, Gougeon Brothers on Boatbuilding, I think it was, a soft-cover, 120 page book of various ways to use their products. There is a section on repairing delaminationed hulls and decks. I looked for it on their website and now it's hardcover and they want $40 for it. Yikes! Try to find a copy at the library or through a friend.
Your boat, if she's still minimum weight (700 lbs. all up, including spars, centerboard, rudder and tiller, but not ground tackle) and has oval spars is still completely competetive. Please keep in mind that any structural additions to the boat will add enough weight to destroy her racing ability and therefore her resale value. If you must add flotation, airbags have the advantage of being light weight, easier to install than building in a tank and are removable for racing.
And with any one design boat, the first thing I'd do is get in touch in touch with my local fleet, even if I was a non-racer. They are an excellent source for advice and support. There are two fleets in Ecuador, one in Guayaquil, which also appears to have some nice, sheltered water for beach cruising a Lightning. Their contact info is on the International Lightning Class Association website.
Good luck!
Paul