Just find one at the junkyard with the towing package, and get parts off it.
Going up hill, it's pretty much just mass, gravity and power. Lower mass would help. Less gravity, but that might make it hard to breathe. More power means more air and fuel in the right ratio. Might be able to reduce driveline losses a little bit, but I don't think there's much low-hanging fruit on the Metro, driveline-wise.
Have you thought about putting another engine in the back of the car?
http://www.carlustblog.com/2008/02/car-lust--twin.html
Or maybe from our very own MetroMPG:
http://ecomodder.com/forum/showthrea...ailer-201.html
Normally aspirated engines can usually be modified to make more power, but it can get expensive, what with porting/polishing, exhaust, modified intakes, etc. I put a Weber on my wife's 1.5 liter Tercel, and that seemed like just barely enough engine for the carb. If you're looking at motorcycle carbs, match the horsepower, not the displacement, and it should work reasonably well. (For instance: a 650cc motorcycle engine making 65 hp at 8,000 rpm needs about the same airflow as a 1000cc car engine making 65 hp at 5,000 rpm.)
Nitrous could work, maybe, as long as you don't need the extra power for an extended period of time (and have a ready supply of spare engines for when you blow this one up).
As far as hitching a trailer to the roof of your car, I can't think of any reason it wouldn't be a disaster. But I'm known to lack imagination.