Check out this thread - it's a couple years old, some of the info may be out of date:
viewtopic.php?f=21&t=2782&start=0I'm sure there are other posts here that cover this topic as well.
My bike runs great with the stock 28mm carb. I had to replace the needle jet, but it was still cheaper and easier than jetting a Mikuni from scratch. It looks like the Mikuni VM26 carb fits the intake manifold and air boot, but is 5mm shorter overall (apparently the VM28 has been discontinued). I don't know about the throttle cable.
Your carb may have problems other than jetting - the float level may be off, the float pivot may be sticking or the valve leaking, something may be plugged, etc. etc. If you'd rather not deal with any of this a new carb may get your bike running better than it is now, but until it's jetted correctly the bike probably won't run too well.
Ray