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.
I know they make gear reduction units that bolt to the side of a small engine and you can buy small engines with them installed. I happen to have a 6:1 unit out in my garage that is a direct bolt-on to a B&S 8 hp engine.
How to you plan to rig up the engine/speed reducer on your pull behind sicklebar? For reference here's my Allis Chalmers 80T seven-foot sicklebar. It doesn't seem like a big job to retrofit this type of mower for ATV use.