Adam
There are a few ways to quickly buy a minsk in Hanoi
Tourists - try to find on the net anyone finishing in Hanoi about the same time there are more riders than you would think so straight to the backpacking area { around the church )
I found two shops selling second hand minsks, had about four in all states of condition, still worth a try. The other side of the lake , walking distance.
Ask your hotel guy to put the word out - I taxied all around Hanoi to meet Vietnamese selling their bikes (I got lucky with a tourist) took me four days hassle including a day buying tyres, clutch plates etc.
Try minsk clubs like minskclubvietnam.freeforums.org • View forum - Buy , Sell or Trade your Minsk and drop a line in the forums There are a lot of guys in Hanoi who love their minsks and old hondas a sort of retro fashion. ( they take pride in having a bike with a clutch )
Hanoi is booming and any number of second hand hondas are available some almost brand new( expect little change from a $1000) Remember that you cannot buy a brand new bike unless you get in cahoots with a VN guy, more trouble than its worth. Again if your strapped for time this would be the go as tyres are about all you would need and they are more reliable, with parts and mechanics every mile. And no hassels with two-stroke oil ( A bugger when you run out and no-one speaks English).
I got good info at a rooftop bar on the main square, where a lot of expats hang out ,you'll find it.
The Minsks best quality is that it is tough, it has a steel frame easily welded and can take the rough roads ( they were copied from a Kawasaki scrambler) and if you get a good one ($350 max) no worries, but its a gamble , the guy I bought my bike from had used engine oil as two-stroke oil all the way from Saigon but I got 6000 klms without problems. Also I saw other tourists with minsks having a hard time. That said I would buy another one, dont ask me why just a soft spot I suppose Oh and they're cheap
Good Luck mate
PS here's a shot of the sticker Lao border guards put on your tank DSC00004 | Flickr - Photo Sharing!
PSS A suggested route If you leave from Saigon with only a month would be to head north to Dalat then cross into Laos at Boy-y, Bolovan Plateau , Thousand islands, then into Cambodia ,Angkor Wat Pnomh Penh then back to Saigon accross the delta. Try to do no more than 200 klms a day, this is not a place to ride when tired,on that basis you could cover 2-4 thousand Klms if keen