Super easy. It's been several years since I modded mine but I believe you can just drop the happ parts in without any cutting etc. I used a competition stick and pushbuttons.
It's realitively easy, but tedious. I did it for mine, and for a friend of mine. The worst thing is crimping all 24 (or 30) fucking 22-18awg .187in-female quick-disconnects. It wrecked my hands for a week afterwords.