So how do we rule and roll for Orks looting vehicles and weapons and such? What would the difficulties be for the tests?

You are asking for a hard and fast rule on how Orkz loot vehicles and weapons? The race that is almost incapable of producing two things identical to each other?

Well good luck with that, but I'll try to give you some pointers at least.

For Vehicles you're pretty much gonna have to ask your Mekboy (or ask yourself if this is for NPCs) what modifications he's making.
If you need a reference point I'd tell you to check 3rd Edition tabletop 40k and see the Looted Wagon rules there.

As for weapons... just use already existing weapon profiles and add Kustomizations (Into the Storm/Hostile Acq) to them if you want the quick and lazy approach. (Don't forget to make anything Ork-built Unreliable.)

Wait then, so Gork and Mork are Underpants Gretchins? That makes so much sense, now.

Don't forget the Red Button.

Oh, and the 'splosives.

Or connect the Red Button to the 'splosives.

If the Ork in question is an NPC then I generally just make a couple of random improvements, a few outright changes and some random penalties and say: Jobs a good un!

In the event of a PC Ork I let the player tell me what they're intending to do and then have them roll a willpower check and a sanity check.

If they pass the willpower check they stay focused and get to attempt to do what they were trying to do. If they fail then they get distracted and end up doing something else, usually weaponising whatever they were working on to en extreme (and often impractical) degree.

If they fail the sanity check then again what they intended goes out the window as does all pretense of making something that will actually work. Sometimes (being Orks) they'll succeed anyway and create something that does work even though it shouldn't.