How do I remove starter key assembly from john deere lawn tractot

Can I assume you mean the igition switch wher you insert the key? Look behine the panel the switch is mounted on, should be a large nut ring, lefty loosey http://www.amazon.com/Ignition-STD36.../dp/B001UL3E74

What model

Where can I find a generic elec schematic of a lawn tractor safety interlock system

A brand or model would help. No generic system, many many different systems used on tractors. I own 3 and each is different. Usually a kill switch on the seat so you can't get off while blade is engaged, another on clutch/brake so you must depress the pedal to start, another stops you from backing up with the blade engaged, 2 of mine have this and one does not.