If all the seals are done, vacuum hoses connected and carby diaphragm in place and it's still overrevving then theres only one more thing it could be...... the bottom crank seal is shot. That would require removing the piston/conrod assembly and the crank and bearings. But before we go to that extreme there is a basic check you can do. Look at the underside of the mower around the crank and see if it's wet and oily, or just sweaty. Sweaty is ok, wet and oily is not a good sign. A photo for the rest of us is a good aide and removal of the blade disk will help coz being a utility it will be hard to see, otherwise try getting some light through the side chute.
