I'd start with a XT 500 aftermarket 36mm mikuni, though if you are going to use the standard airbox you'll need to check external dimensions. Using an airbox tends to give you more low down power.
The suzuki engine probably isn't any more powerful than an XT, test reports of the time didn't think it was much different to the DR400, as far as power goes!
So the overall breathing requirements shouldnt be too different.
