Theres no such thing in the world as a Mikuni slide carb built for a Briggs 5 horse rototiller...:lol:
But yeah.. 20-ish millimeter (so the air passage is about right) will work pretty great on one... You're just gonna have to come up with a main jet that works to your liking.. Probably ABOUT any size you could want is available...
But I have noticed when way over jetted like with pre jetted carbs.. You can run a single fine piece of copper wire through the actual hole that IS your jet, and keep stacking one after another, effectively plugging the hole off, restricting fuel flow, and leaning out the carb.. :shifty:
But pretty well any 20 MM Slide carb is gonna go on like a 125 dirt bike and be DUMPING the main fuel to ya.. :laugh:
I would definitely not go over like 20 on a stock engine.. If you go bigger than it can handle at idle, it will be kinda sluggish to respond to throttle input.. It will cause ya to cough out under a massive dump or air when ya punch it wide open... A slightly restricted center bore will help it pull fuel at an idle speed, and allow ya to snap the rpms up faster, due to it has plenty of fuel mixed into the stream already....