It doesn't have to be a Narco antenna! Narco ELT's used a passive antenna (that's basically just piano wire cut to the proper length), so ANY passive ELT antenna will work just fine with no detriment to operation. If it fits the hole, just mount it and hook it up to the BNC connector on your coax.
I suggest the one below as an inexpensive solution (using an old antenna is possible of course, but how long before IT fails? I've lost two that left their mountings!).
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http://www.aircraftspruce.com/pages/av/antenna_elt/ak450eltant.php
