Dog has a Diagnostic Port!

Working through (I say working, more like blankly staring at) the engine bay I noticed this little chap - cover that says "Diagnostic Port". I'm assuming that Dog doesn't have a computer onboard, but if it did, considering how long it hadn't been used, and the way it's running right now, it'd be listing a bunch of errors!

Plus it'd probably be a bit sore that there are no emissions control on the engine whatsoever. Every part of the emissions system bar the exhaust is missing. Not needed in this state or it's previous state apparently.

Right now, I have no idea where to start with this engine to try and make it run just a tad better than it does.