Are there any rules on miniaturization of vehicles and arms from small characters or mini-drone versions of vehicles?

There are general miniaturization rules in Spy games.

This may work, thanks!
And are there vehicle upgrades that give groundcraft aircraft options? Or drones with such features?

There is a flying mod for vehicles in Arsenal

What's the name of it? I've found gliding system and hovercraft upgrade, but no aircraft or rotorcraft upgrade

Well there is the LTA blimp upgrade but I don't think that's what you're looking for...
I think you can take an aircraft and modify it to be a ground vehicle however

Now I need to get my hands on spy games. I'm going to take a Main Battle Tank and make it as small as it will go...
Or maybe I'll try and shrink down a Gauss Rifle to the size of a holdout...
After all, if it's worth doing it's worth overdoing.

In Arsenal too?

Yes, in "Arsenal".
I'm not entirely sure about miniaturisation of firearms, but do as you will.

Still couldn't find it ( Can you point to the page in question?

You're actually looking for a modification called "Lighter than Air" (in case you didn't know) and it's on p. 139, "Arsenal".

Ah, that one. Thanks!
i thought there was a way to make groundcraft a regular aircafrt, not LTA blimp