O H€££..... I thought you were gonna use the whole snowmobile drive. Well in that case, got TONS of ideas. Find a set of used A arms from a quad. Get the hubs/brakes, and all..... MEASURE the distance between the two arms, @ the frame, for future reference.
With that size engine, you can EASILY make this a small/skinny 2 seater, as it will have PLENTY of power. Just saying, some solo riding gets boring....
Now we are at the point of deciding if you want a stationary engine (I would) or one mounted to the swingarm. This is gonna determine the shape of the frame.
Now, being that the recipient is handicapped, I envision a frame simular to my sandrail, but a bit lower on the main side tube, and then adding a higher tube . The higher tube will be mounted on a thick plate hinge on one side and either a safety locking pin or latch on the other, by the seat. This bar will open outward only so far, to act as a grab bar to assist getting in and out. Once in, he pulls it closed and locks it...... this will enable you to have a high safety bar, yet facilitate egress/regress..... Just an idea......
And now, for controls, got to be on the steering wheel, both throttle and brake. For brakes, got to be hydraulic disc in back, the brakes from the atv set up you bought (as advised in first part)for the front. Use a steering wheel like this would make it easy. The brake res will be mounted on kart, actuated via cable.
Need I go on, or is that a good start?