If you can get them to put the 1430 wheel motor boxes on it, like JD Beach did, you can put MUCH larger wheel motors on it than the current 14.3ci. Keep in mind, though, that the lube system on the Robin is only rated to 20 degrees, while the Kohler is rated to 25 degrees.
I first upgraded my Mossroad-vintage machine to Char-Lynn S series 21.7ci wheelmotors. That required that I cut a hole in the tub to recess them into the tub slightly. They worked well as far as power, but the taper on the shafts is not the same as the old 1" wheel hubs, and it eventually wallowed out the hubs so badly that I couldn't keep the hubs tight on them (the metal of the PT-supplied hubs is VERY soft). I do NOT recommend using these wheelmotors with tapered shafts. Some have had success with the keyed, non-tapered shafts and corresponding hubs. But finding 5-lug hubs for 1" shaft wheelmotors is a bit of a challenge...
Two years ago, I upgraded once again, cutting off the old wheelmotor boxes, and bolting on 1430 wheelmotor boxes. I'm currently running 28.3ci with taller 26x12x12 lug tires (similar to JD Beach's tradeoff though his are turfs) and 1.5" tapered hubs -- which essentially cuts top speed in half, after the taller tires, but gives you twice the torque. Wheelmotor torque is no longer the limitation at all -- only traction. At $169 each through
Surplus Center, or $179 with hubs already on them the wheelmotor boxes cost almost what the motors themselves cost. They stick back into the tub about flush with the inside edge of the tub -- protruding less than the smaller Char-Lynn S-series I tried previously.
28.3 cu in HYD WHEEL MOTOR W/5 BOLT HUB
My limitations are no longer wheelmotor torque, only traction, and I routinely use mine on 20-25 degree slopes...
Given the lubrication system limitations of the Robin, I think a 21ci to 23ci wheelmotor would likely work quite well for you, and likely would fit neatly inside the 1430 wheelmotor boxes without any cutting of the tub required....
My experience -- your mileage may vary...