Did you know that it was named DECCA after Bing Crosby persuaded his record company (DECCA) to take on the development of the system to provide accurate navigation for the D-Day landings. I believe he may even have invested in the project. Thats the short version of the story.
Keep your unit in the loft it will be antique and worth thousands - one day!
Depending on the model there is (at least was) a GPS antenna conversion available that receives GPS signals and then converts into the signal required as input to the Decca receiver.
Maybe that helps? It does of course mean spending more money, but then you may be able to get one second hand....
If it's a MK3 there is nothing that can be done other than bin it. If it's a Mk4 or 5, ie one that can accept nmea input then AP make a gps receiver called a survivor that can be connected to allow the decca unit to run as a gps. Try comar on the Isle of Wight for advice etc.
There was a request a while back for old Decca units that could be converted for blind sailors. I seem to remember it was to do with the way they beeped.