Not so fast, how will you reduce the speed? Chains and sprocket or pully and belt. By reducing the speed, you could amplify the torque by a factor of Four plus. What type of mower do you have? Pitman arm? 7 ft or 9 ft?. I operate my NH 451 w/ a 21 pto hp ford, no issues, I should look in operator manual for min hp requirement..
Be sure to use an "Overhung Load Adapter" on the engine output shaft to protect the shaft bearing. Available from surplus center.