Motorcycle carbs are usually slide throttle type and dont mount with a flange like most the carbs you see on cars (like mikunis on ka24e manifolds).
Mikuni 44mm sidedraft is the japanese version/alternative to the weber DCOE. Some (a lot) of people prefer the mikuni because it stays in tune and/or easier to tune (preference). 48mm+ mikunis are rare.
The weber was designed long ago in europe, the sidedrafts you see are known as DCOE and have been used on european sports cars for ages.
Solex and SK are a couple of other manufactures of DCOE type sidedraft carbs. Solex is no longer made but you can get SK's new.
EFI throttle bodies designed for DCOE type flange pattern will work with these manifolds.
sohc manifold
Most of the ka sohc at this link are using this manifold, sold by nissan motorsports and made in california by B.C Gerolamy.
http://hybridka.com/kagallery/gallerypg1.htm