What brand of light? I think O'DAY was using PERKO lights, so a check of their web-site might help,
www.perko.com (I think?) If another brand...... check that brand's site, or A call to D&R Marine may be your best source of that info (assuming the light is the original), Rudy Nickerson at D&R was the Parts Dept. Manager at O'DAY from 1973-89 and now runs a little shop supplying replacement parts for our boats. He may just happen to have an extra lens lying around his store. (508) 644-3001