You may start with talking to the dealer or Classic Center. A 000 760 04 06 is the master key part number for 113044 until chassis 0 12689 and it is available. There are several other part numbers - some available, some not, but I would try there.
However: the original master keys from the dealers are of not the best quality, to be honest. They have three parts, you put them together and they do not hold very well. I found much better replacement keys at a corner key shop.
Lots of information in our Technical Manual about the keys...