Unfortunately, I would have to have the machine in front of me to give you numbers, but first we have to put things in perspective. I am going to guess that your spreader loaded weighs maybe 2000 lbs. it is ground drive, so it needs enough traction to drive the beaters and apron. Probably 75% of the power is used by the beaters. If your tractor develops 35 horsepower, the apron uses a small amount of that because it is also powering the beaters, pulling the spreader, and moving itself. If it were mine (ingenious use of an old spreader by the way), I would get about a 5 hp engine and a reduction gear of some kind and power it that way. That would give you a nice range of speed rather than being limited to one,,,,,,just throttle the engine up and down.

That would also help you design a hydraulic drive. A Predator 212 engine develops 6.5 hp at 3600 rpm and about 8 ft lb of torque at 2500 rpm. If you want to run your apron at 25 rpm, it would require 800 ft lb of torque with a 100 to 1 reduction gearbox.

I would suggest you have your lady count the revolutions' of you apron sprocket as you run the machine and start from there.