Check on the side of the engine block for a rectangular steel box-shaped affair, with moveable pieces inside it, This is the throttle/choke mechanism. I'm not sure if your engine has the same mechanism as the older engines, but you should at least check the throttle cable adjustment, to see if maybe it needs adjusted. If the cable is not set right, it will cause high engine speed.
Also, the inner part of the throttle cable, the thin wire, might be broken inside the cable, or an end might be broken off. Also, check up behind the dash panel (Instrument panel, to the younger set) for a stuck or broken throttle control handle. The end of the outer coiled part has to be firmly clamped into the throttle handle assembly, for it to allow the thin part to work correctly! Same on the other end!
HTH: Rusty Jones

If you can reduce the speed by holding the throttle shaft, then chances are it is internal. Obviously, you can inspect the linkage and rule that out first. Engine #s can be helpful in a case like this. Some twins had bigger problems with the rod that the gov sets on in the block.

Governor problem. Model 407777. Internal governor came apart causing a high rpm problem. I got a new governor but it seems to be a different set up. P/n 793338. Could this be a new design or is it the wrong part. Any help would be appreciated. Do I have a side valve or a ohv motor.