Go to your nearest bike coop, old spindle in hand, and buy a replacement that is dimensionally the same. Maybe more likely to find a square taper, but running across a cotter spindle isn't impossible.
^^This. Our local coop keeps an ample supply of both pottered and square taper on hand.
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My bikes: '81 Trek 957, '83 Trek 720, '85 Trek 500, '85 Trek 770, '81 Merckx, '85 Centurion Cinelli, '85 Raleigh Portage, '92 RB-2, '09 Bianchi