Advise the police that you've recovered it for safe keeping, and give them the details for their found property register.
As a practical matter if no one claims it within a reasonable period of time, and you've made reasonable steps to identify the owner by notifying the police and asking around, then you can assume the rights of an owner with a clear conscience.
Technically, however, if the owner comes forward in five years time you must return it. However, this is unlikely, and by then you'll have doubtless fitted new parts so you will have a financial stake in the bike yourself and as such could be a legitimate co-owner, so they've need to sue you for its return if you chose not to do so. Very unlikely indeed.