Sealing the buoyancy tanks can be a tricky task, but you need to do it to get a measurement certificate for class racing.
There are some good articles online about various methods for tracking down the leaks.
However as a quick fix I would definately suggest installing some buoyancy bags or whatever inside the tanks. Some people use the plastic bladders from inside boxes of wine, possibly because it is quite enjoyable disposing of the original contents.
I think the two tanks are supposed to have a certain amount of foam buoyancy installed anyway, by class rules- about 40l per tank IIRC. This is to help prevent the boat from foundering should the tanks flood. However it would not be enough to keep the boat floating high enough to be recoverable.