Consider yourself lucky if all you need are new gaskets. I bought a 1969 LS-13 a few years ago and had the same problem. A little investigation revealed one of the few weaknesses in the LoneStar 13/16 design... the fir plywood cores in the leeboard trunks were insufficiently isolated from water. I suspect this led to the demise of many otherwise solid Lonestars out there. When the plywood swells with moisture, the hastily layed-up fiberglass trunks may crack or partially separate from the rest of the hull and the screws in the aluminum mounting flanges for the pivot levers will strip out of the rotten wood cores.
The best long-term repair is to remove some fiberglass and excavate all the rotten wood, replacing it with either a strong waterproof core material or well encased quality plywood. If the mounting screws are still threaded into solid, dry wood, all that is needed is to ream out the exposed wood around the hole for the pivot shaft and seal the new bore with epoxy.
Then you can make your own pressure plates & gaskets out of 1/8" thick UHMW Polyethylene sheet and a properly sized O-ring. The polyethylene can be cut using a scroll saw or CNC. The O-Rings should be sandwiched between concentric rings of UHMW and a pressure plate of the same material, and can be adjusted to provide the correct amount of friction to hold the boards in any position.
I also had to cut away the inner shell of the bilgeboard trunk around the core and replace the plywood. The core swelling and sloppy factory assembly required me to repair cracks around the base of the trunks with epoxy fillets as well.
It was well worth the effort, because my hull has been bone dry ever since, so long as I keep the lee rail above water when heeling and remember to install plugs before launching.
I am hoping to get a Chrysler LS-13 that is being given away. I spoke with the owner and he said that it needs repairs, possibly the bilge boards or where they tuck up in. (from what I could understand). I'd be interested in any information about repairs for this boat. Thanks