Your motor is designed to run 3600 rpm. That is where your engine is set to run with the governor when it left the factory. You can possibly safely tweak a factory engine to 4000ish rpm safely. Beyond that the rod is going to fail and quite possibly the Chinesium flywheel is going to fail.
You cant combine the governor with the mikuni because there is no way to hook it up. Slide carb vs a butterfly carb. It's just not gonna work.
If you want to run that mikuni get a billet rod, billet flywheel, cam, heavy valve springs, and your mikuni. Then you can spin that thing to 8000 rpm and rip like a monster.
If you don't want to do all that then put your stock carb on and hook the governor back up and leave it alone.
There is no free horsepower. It all cost money. I'm not sure where you are from but if you want to save a lot of effort and time a tillotson or a ducar clone will run 5500ish rpm out of the box. Couple more horsepower than a stock clone.
The only reason I'm wasting my time trying to explain this is to try to save you a motor. I can't tell you how many times on here someone posts their engine won't run anymore. Then after wasting time trying to help them they say, oh i removed the governor from the engine. Well your engine has blown up and now you need a new one. Good way to waste 150 dollars. But at the end of the day its your engine and your safety. Do what you wish but no one here is going to tell you to send it that way.
watch this video
it will explain how the governor works and if you can envision trying to hook this up to a mikuni carb you can see why it will not work.