You may need to rebuild the rear end of most small bikes to fit a wider tire in there. And you won't need rim brakes but you will need disc brakes and mounting tabs, which requires a special jig to braze it on in the correct fitting. Ask at a bike shop who has one. See the rear fork made of rectangular tubing.
For a really heavy duty tractor to haul a large trailer, think about very low gears; You don't need speed. Hub motors are made for speed, so I put a motor with planetary gears built in. A mid drive is good if you need more than one gear reduction. For steep hills I need a second slower hill climbing gear. See my build thread here: Mt.Goat Cycletruck build.
You will probably need some extra tubes from old bike frames to beef up the frame of the small bike. Square tubes or 1.25” flat bar for the front and rear racks. But you can make round tubes work if you think about it. Don't just let the ends hang there because they can start a vibration, so brace the ends. The front rack needs to NOT be connected to the steering fork, because it will effect the steering badly.
This is the best thing for utility cycles, a two speed crank! With a Nuvinci infinite gear hub.