So your plank on frame boat has sprung a leak and you can't see where the leak is. You can suspect that due to age, small cracks have appeared in the planking. The external hull paint work too may be cracked and damaged.
Remedy:- Make sure the hull and wood is thoroughly dried out. Then clear as much of the internal electrics away and make a mix of fibreglass resin; pour the resin, whilst warm into the interior of the hull and tilt the hull every way you can to make the resin find all the interior nooks and crannies.You have to be quick with this, as the resin soon starts to set and won't run. Allow the resin to set and test in your bath or test tank. I have had a 98% success rate with this.
You can try a second application if it is still leaking.