The spark ignitor is the part with the single wire leading to it on the bottom left of the burner assembly. It would have 2 metal rods that are angled to sit over one of the burners. The metal rods can be cleaned with a small metal or diamond file, steel wool or sandpaper (but should be wiped off if you use sandpaper as silica "dust" is an insulator). At the same time the grounding surfaces of the spark ignitor leading back to case ground need to be cleaned also (see page 45 of the manual that ajw22 left the link for you). This means the underside of the spark ignitor plate that has the uninsulated ground rod attached, the part of the spark ignitor bracket that the spark ignitor plate attaches to and both sufaces ( on the spark ignitor bracket and the burner assembly ) where it attaches to the heater cabinet via the burner assembly. Hard to see from the photo, but also any metal to metal surfaces from the spark ignitor bracket back to the cabinet ground must also be cleaned. What you are pointing at on the right hand side is the rollout safety that prevents the heater from running if the combustion side of the heat exchanger is partially blocked/blocked.