Check with the lodge (now merged) that you were in as a youth OA member. The council that is served by that lodge can give out contact information.
Do you have your OA book from back then? If so, you may have recorded the name of your lodge,date of election, call out date(if that took place) and Ordeal weekend dates. Your last OA card would serve as proof of membership.
Once you've gotten the information to your local lodge,they or your council will gladly accept this year's dues and sell you a uniform flap patch and sash that fits.