How to send receipt for previous event sign ups?
You might want to test the following:


Navigate to Administer Communications > Message Templates, select System Messages, then beside Events - Receipt only. Leave this window open and open another to Administer Communications > Message Templates, and click New template. Copy the subject, html body and text body from system receipt message into you new user one, give it a name and save it.
Navigate to Find Participants, select a registration of your own user for testing (you may need to create one first), then for action select Send email to participants. On the next page, select your new template and send the message. If the receipt looks good in your inbox, repeat this step but sending to unreceipted participant.