I suggest to you based on each motorcycle being used for same purpose, that you keep an inventory list (like a subsidiary ledger) of EACH motorcycle, identify it with a unique number, so there's a tracking record of each cycle, and when sending out for a delivery, keep a record of which driver will be using it.
Yes - it will create a lot of work in tracking - but should a specific cycle go bad, maintenance records can trace the specific cycle's usage, who drove it, how well maintained it was etc. I realize that the insurance policy will be burdensome too.