Those boxes are often still usable after a thorough cleaning, but I think that yours is beyond that. A used set with the central locking unit, key transponder, and DME can usually be found on eBay (be sure it's from a compatible year/model). You can also send your box out for repair:
M531, M534, M535 Immobilizer CLU Alarm Box for Porsche Boxster - Specialized ECU Repair.
If you didn't pull the carpet out yet, I'd strongly recommend it. The foam padding underneath holds a lot of water and does not dry well in place.