Quoting directly from my 1985 owner's manual, "In case of loss, duplicate keys cannot be furnished by your Porsche dealer." However, it then says, "If you should lose a key, provide your Porsche dealer with the key number. The key number of the ignition key is impressed on a plastic tab which comes with the key." This is not such good news for you.
However, check these articles:
Lost only ignition key? Don't panic. - Rennlist Discussion Forums
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