I'd be willing to be that the guy who buys this is going to prep it, paint it, and park it in his lawn.
I'd have to agree with "Blue Duck" on this one; you'd be asking for trouble if you modified that mower to pull behind a four-wheeler. You'd either need a substantial re-working of the controls or have another person riding in back to raise the sickle and disengage the ground-drive.
A ground-drive mower on steel. We've had some improvements since then, like tractors, 3-pt. mounted PTO-drive sickle mowers, etc. If you're dying to know what it's like to farm with horses, some of us old timers can tell you the tales, and you won't have to buy a couple teams and the horse-drawn equipment. Most everyone has a "I remember that time the team ran away when I was: on the binder, the mower, picking corn and forgot to use the drag rope, etc."
Sometimes progress isn't all bad.
(Please don't take offense Ken, It was fun to look at an old No. 7, but the thought of that mower behind a four-wheeler made me chuckle.)