The beauty about most bike carbs is the fact that many of them are actually a variable Venturi type. Meaning you can actually run 40s because the throat of the carburettor actually varies depending on the vacuum on the manifold side (the vacuum is dictated by your throttle position).

Keihin CVK are my favourite cheapies but the Mikuni equivalent is fine. CV means Constant Vacuum, which is the type. The K simply refers to the Kawasaki company that enlisted Mikuni to produce them.

I've never heard of matsouka msr but I have heard of Mikuni HSR which are the Keihin FCR equivalent.

Generally you get a piston and throttle valve type, a flat slide and throttle valve type and a straight flat slide type. The prices increase in that order of mention.

You have many, many options out there, aim for carburettors similar to your engine capacity and don't assume that the more expensive, the better. I had a set of pov Kehins from the early 80's and even they were a delight on a stock motor.

Scrounge eBay US and link up what you find. If fitting to DCOE/DHLA/PHH Manifold be sure to confirm carburettor bore spacing. Some/most carb banks come with an adjustable rack so you can spread or close the bank, when spreading the spacing you may need some extensions to the fuel rail but all very simple if you upload some pics.

You are on the right track but there are literally over 100 options! I am a die hard carburettor fan and bike carburettors are a technology that never seemed to carry across to the automotive industry.