You could manually re-enable each buyer's original download link by individual Transaction ID, assuming they even have or can find their original thank-you email with the link to their thank-you/download page anymore.
As an easier alternative, you could just use our Updates feature to issue a new link to previous buyers of any selected product(s), at least those buyers who have not opted-out of joining your Buyer Group list:
http://www.e-junkie.com/ej/help.updates.htm
Updates do incur a nominal fee to cover our additional overhead in sending and fulfilling update downloads. The bottom of the updates page explains the fee structure. This amount is calculated automatically when you choose to issue an update, and presented for you to approve or decline the payment before the update is sent.