The motorcycle parts can be tough to find, I'll describe them here. I advise that you rescue the majority of the survivors on a separate playthrough as time is tight.
- Engine - Alleyway behind the cinema.
- Gas tank - Directly outside the safehouse, at the petrol station.
- Wheel - Purchased at the pawn shop. You need to rescue the owner, he will be adjacent to the safehouse after you find the zombrex vial.
- Bike Forks - First get the shed key from the hotel's second floor, you need to jump up onto a fire escape to get up there. Then head over to the shed, it's on route to the quarantine zone.
- Handlebars - A survivor has these (located in the parking lot near the cinema/department store) though he won't give you them unless you trade him a broadsword. Either kill him or use this video to find the sword