Funny how a tiny chunk of gravel or itty bitty piece off a log will bring a wheeled item to a halt...so you need a large wheel. Then you need to find away to keep the **** thing from "walking" on its own.
Your dad is not getting any younger so think long term. Mount the generator in a shed that can be opened up for safe operation. Have the natural gas line run with a ball valve so he does not need to connect it. Run a 110V outlet for the trickle charger and wire up a couple of lights. Sink a pole and chain the generator to it. Have a good lock on the shed door. Dragging out a generator and hooking it up at night and/or a storm and/or a foot of snow and/or a sheet of ice gets harder and harder. Seems we rarely need the gen set when it is pleasant out.