First, there's probably a pressure relief system already on the tank.
The problem is that is analogous to drilling a hole in a pressure cooker, it's either going to rupture the tank entirely, or blast the contents into the air.
Also curious where you would get a "quadcopter with a small drilling machine" with a couple hours' notice; even if you had one, it would be hard to maintain a position at one exact spot, and much of the lift energy would be expended in the torque reaction of the drill.
The only sane way unless you have not enough water to fill the tank. Oh wait, you can fill it with that solid water on the ground, as long as you give it a few hours to melt from the ambient heat in the house between each use.
The gear teeth are cut with a file. For the angularity, draw a circle with a compass and subdivide it by measuring linearly with a measuring tool. This can be done larger than the part, and the teeth locations marked with a straightedge. By cutting the teeth where marked, you avoid a stack-up of error.
Traditionally, lack of demand and the fact that GM was fastidious about keeping them off the road means that they would probably threaten a lawsuit. Electric cars in general have only become popular in the last 5-10 years; the lore of the EV1 has grown accordingly.
Copyright law for art and sculpture requires registration of each design; in searching the copyright records it appears that GM doesn't do this. Really the more appropriate forum would be to get a design patent but those last for only 15 years anyway.
Trademarks must be registered (and also apply to specific categories, though a kit car and production car are in the same category). Surprisngly, "EV1" is owned not by GM, but by Kia (the graphic is different). What this means is you can make the (GM) EV1 logo no problem, and also sell a kit car as something like "inspired by the GM EV1" but if you sell it as an "EV1" then Kia might come knocking.
In short, I don't see much getting in the way of making an EV1 kit car as long as you don't advertise it as a literal GM or EV1 car. Though as stated, you can include or sell separately an EV1 badge that buyers can slap on their own property without issue.