Yes the leaks can be fixed
A little more info on where the water is entering the cabin would help. The dorades forward of the mast on the early E38s are constructed with plywood floors that are part of the deck core. So these areas are built-in leaks that may never have been sealed. Despite being covered and cowled the dorade boxes can get wet inside. Water in there could wet the core of the deck over time and could creep around in the core fore and aft and find a way into the cabin in a variety of ways. The space between the dorades, if it has a long round ended opening, has a plywood floor also with the same issue - it is resting on the lower layup of the cabin roof and is therefore connected to the core.
The tubes that carry the air into the cabin, entering on either side of the mast, are usually PVC that is glassed into the plywood floor. The resin and glass don't stick to the PVC very well in some cases and have to be dealt with. A leak there could lead to a leak around the vent holes in the cabin ceiling, and also wet the deck core.
Do a search on the forum for dorade and you'll find a number of threads that might help. I have a blog post on dorade area repairs and am still in the middle of a remedy.