Im looking to buy an old motorcycle,fix it up, and go across central asia.What bike would be most suited?

The bike you typically see in this kind of situation is a BMW GS model. They started making these in the 1980s and they have been very popular ever since. It's an on/off road model with long suspension travel and brackets to protect against rocks flying up from the road, etc.

In the old days (the 80s) they made an 'airhead' GS, that is the old air-cooled flat twin. In the 90s they discontinued the airhead and began making the 'oilhead', a more complex, more 'modern' flat twin model that had oil-cooled heads. The later model is more powerful but also a lot heavier. There are a lot of both of these models available used, and once they're in good condition they are totally dependable, practically bulletproof. There was an F650 GS, a single-cylinder 'entry level' bike, but I think that would be too small.

"Central Asia"; you mean, like, Kazakhstan, Tajikistan etc? The BMW idea is okay, I guess. I read an article in a bike magazine recently about two people who did it on Suzuki DR 650s. (They didn't break down, so didn't need fixing!!) But in that part of the world you should also consider a Ural or Dnepr. The links are to the websites for their "new" bikes, but as far as I can see, they're the same as they were 40 years ago, and they come from that part of the world so the locals will be familiar with them if anything goes wrong.

Have a nice trip!

(Sorry – KMZ/Dnepr site is not available in English)
Urals are available as solos, although you wouldn't be able to guess that from their website.

Man travels the whole world in search for happiness and returns home to find it in his own backyard.

Save yourself the trouble pal, unless you have an iron butt and a stainless steel spine.

This idea of an old motorcycle is all wrong. You need a brand new bike, properly run in of course with all the modern day tech to make your ride that much easier and smoother. You need it.

A Yamaha MT-03 would be a simple but rugged companion.

BMWs are the default choice, if you can get one the R80GS would be ideal (they don’t often come up for sale). Honda C90 Cubs have been used to travel the world if you want to take your time.

The Mongol Rally used to use some very old cars to travel from Europe to Ulan Bator – 2CVs were popular (Mongol government now demand cars less than 10 years old).

http://mongolrally.theadventurists.com/index.php?mode=gallery

An old Suzuki DR350 Enduro would be perfect. Rugged, light, economical, cheap, single cylinder air cooled, easy to fix. Hard to beat that.

What many people don't seem to realize is that BMW is a bit like Harley Davidson or Ducati. They are great in the western countries where money is plentiful. But nobody buys these bikes in less well off countires. You would be hard pressed to find even a tire for these bikes.

The Kneper and Ural is a good idea, not that they are a great bike, but locals can help fix them along the road. The local mechanic on the roadside in the stans is not going to help a BMW GS1200 with a clogged fuel system due to the poor quality of dirty fuel.

Besides, nothing says 'please rob me' more than struting through an impoverished country on a $20,000 dirt bike.

You could do it on a Nimbus, like these guys:http://www.advrider.com/forums/showthread.php?t=419271&highlight=nimbus

But the trip can be so rigorous that you are probably better-served by something newer. In any case, you do not want to make the trip without reading Walter Colebatch's accounts of his journeys across central Asia and Siberia.
http://www.advrider.com/forums/showthread.php?t=545121
http://www.advrider.com/forums/showthread.php?t=533442
http://www.advrider.com/forums/showthread.php?t=463661

It does not seem to matter too much which bike you use, except that the 700-pound GSes that Ewan and Charlie rode were way too heavy. The R80GS never got 55mpg, more like 35,and is also too heavy, in any case, although it is probably the best boxer to take.

A single from 350 to 650cc would serve best, IMO, and based on the reports of others. My own choice would be either a Suzuki DR650, or a DR350. A bigger tank and aftermarket seat is indicated for either one.