A great resource is "Breaking New Ground" out of Purdue University. They should be able to refer you to companies that specialize in adapting systems for tractors. I saw a company at the Indiana State Fair that was displaying a John Deere 4710 tractor with a seat lift and all hand controls. The name of the company escapes me right now, but the people at Purdue should be able to give you that reference. You may find what you want on the web site, but I would suggest giving them a call also.
Breaking New Ground Resource Center