Small Engines (Lawn Mowers, etc.): how to converta lawn mower engine to a go kart engine, centrifugal clutch, vertical shaft

Question
how to make a 12 horsepower lawnmower engine work for a go kart

Answer
I need more information on the engine.

Horizontal or vertical shaft.
Do you have a go cart clutch or centrifugal clutch.  
Is the fuel system gravity or fuel pump.

It may be cheaper if you go to a go cart shop and look at one.  Try to visualize your engine .  Where does the choke attach?  Where does the throttle attach?  Where does the fuel tank sit.

With that big of an engine you may be to light in the front and not be able to control the steering under speed. The clutch is critical/  It tells the drive axle to turn when it reaches a cretin RPM.

You need to do more research.