**** TOO LATE ****
I would be very hesitant myself to change a plug without having received instruction in doing so by a certificated mechanic.
Legal does not always mean wise, FAA-land or EASA-land.
Tinkering with the ignition on my car I dont mind, if it stalls at the end of the driveway it wouldn't kill me.
Just a future recommendation; there is a bunch of stuff you are legally allowed to do as an owner. Take this list and decide what is likely to occur to you considering the type of flying that you do, unattended farm strips.
Take some instruction from a mechanic and carry the basic tools to do:
- Tire change or tire repair
- Plug change or cleaning
- Oil change
- Basic fabric patch
- Battery charging or replacement
- Landing/taxi light replacement