The OP should create a MYCIC account and link the letter sent rejecting the ETA to that account using the reference number.
Then print off the renounce form, fill it in and attach a photo in the space provided, scan the whole completed document plus a scan of passport bio page as ID as a single PDF document. Then from within the MYCIC account upload the scanned document to the link in MYCIC to the ETA application and if lucky within a week or so should get an email approving the ETA.
As ROB TO said for any information do not have just write a letter of explanation and include it with the single scanned PDF you upload. Even for any dates on the form put approximate dates and for passport just put current passport details.
Ultimately for a renounce all immigration want is the form with a signature and ID confirming you are renouncing all rights as a PR , the rest of the data is really just backup. A few cases on here that were PRs 30 years ago and forgot, they for sure wont have any original information to complete the form but still succeeded in renouncing without much fuss.