To the best of my knowledge, the 2003 Start did not come with a removable center fin.
It came with a short thick centerboard that stuck up out of the deck in an EVA channel.
Does your Start have a Tuttle fin box in middle or a big long slot for a centerboard.
If you have the fin box, then any std. Tuttle fin around 40 cm in length should work well.
If you have the retractable centerboard, perhaps they have some old parts at Starboard
HQ in Thailand.
First confirm that you have the center board type Start and perhaps we can assist you
in getting in touch with the right folks at SB HQ.