I would think a speedometer drive would be easy to locate on fleabay , Though I havent been watching for them . There a lot of 1500 parts listed .
Air pressure is going to depend on the weight of the passnegers and how soft a ride you want . I am about 250 lbs and run about 50psi . That may be to harsh a ride for you . Its best to test a few times and find what works for you . You will probly get some more answeres though from folks who ride 2 up all the time.