Job number 1 is to make sure you have ownership and no duties/debts/losses owed by the bike caused by previous owners or commitments to previous owners.
Would be a real shit to get work done only to find it was a total write off a few years ago, or you get it finished register it and then the police turn up, thank you for your efforts and take it away because its stolen property.
It is literally job 1, i cannot emphasize this enough.