Do you know if it's possible to rent a motorbike in Hanoi and return it in Sa Pa?
Hope this reply is not too late.
There is Thang Pham's motorcycle shop at 15C Cho Gao and he has many motorcycles to offer you.
Here is his contact : +84962309988 Mr Thang
HN- Sapa is around 360km. You should take a sleeper -bus to come to Sapa. Then rent a motorbike (80k- 120k/day) to explore Sapa, visit the ethnic village, waterfall...