The best and most reliable Minsk rental in Hanoi is Cuong’s Motorbike Adventure. They are conveniently located at 1 Pho luong Ngoc Quyen (about 300m north of Hoan Kiem Lake). For the exact location on the map, check the Old Quarter map in your Lonely Planet Vietnam
Update July 2010: Looks like Cuong’s website is up!
Cuong’s Motorbike Adventure
Cuong has a fleet of Russian Minsk motorcycles. The Minsk is a two-stroked motorcycle and requires mixing of 2T oil with petrol. Petrol kiosks attendants will know what to do with your Minsk, but just bear in mind that they should mix around 500ml of oil for every 10litre of petrol. The motorcycle does not comes with an electronic start, so you should be adept at kick-starting the motorcycle. The solid suspension and tractive tires of the Minsk make it suitable for the Northwest mountain roads. Its popularity in the mountain areas also makes it easier to find mechanics should you meet with any bike problems.
There are two models of Minsk in Cuong’s fleet. The differences between the classic Minsk and the sports Minsk are that the latter comes with a larger petrol tank, a higher seat, a higher front suspension and a larger engine block. Cuong rents out both models at US$6 per day. Should you require a helmet, you need to pay an additional US$1 per day. Luggage bag is available for rental at another US$1 per day. Cuong also provides the following with any Minsk rental:
- Lock and keys
- Papers on road advice, driving tips and Minsk maintenance
- Minsk repair manual
- A set of basic tools
- Rubber hooks for tying your backpack to the back seat
Russian Sport Minsk
As a deposit, Cuong requires your passport. When you get back from your trip, he will return you the passport and you pay up. This is the standard procedures for most motorbike rental companies in South-East Asia. If you want to leave cash in place of your passport, the amount is usually the price of the motorbike.
Regarding the requirement of a motorcycle license, Minsk Club Vietnam reported that an international driving permit is not sufficient and law enforcers have a right to fine you for riding without carrying a valid local license. That being said, rental shops will still rent you their bikes even if you don’t have a local license. Many tourists have rode without a license. If law enforcers do stop you on the roads, I am sure you would not be able to understand their language and have no choice but to walk away. 
Besides Cuong, there are other motorbike rental companies in Hanoi. Here’s one I found from searching the internet: Offroad Vietnam