Insert a better engine.
Seriously, this question has been asked down the years many times, I've been the asker myself, and there's not really much you can do to it that will be better in terms of pound-per-horsepower than just dropping in a whole 1.3GT lump.
If this is to make it go quick whilst avoiding the insurance bump, chop it in for a 1.0, 60hp, group 1 Corsa. Otherwise, you stack it, they notice the undeclared switch, your policy is void and you're in a whole barrel of legal and financial pain.
One thing that improved the nature of the engine if not the power was one of ........ oh man, it's been so long, I forget his name! There is or was a fine upstanding chappy on here who does custom ECU chips. I might have got one or two HP bump out of it and a little extra torque (enough to shave half a second off the (20-ish second) 0-60 and maybe do 91-92mph instead of 90 - nothing significant), maybe some better MPG, but it definitely got rid of a couple of noticable flatspots in the lower-mid rev range. Drivability is more valuable in a slow car than absolute power - if you can hold the gear without having to continually change, it makes zapping down the B-roads at 50-60 a much smoother experience.
You could put in different cams, manifolds, throttle bodies/injectors and the like, at random cost and a fair bit of work, and end up with a frankenstein engine that will baffle the average garage and put out maybe 60 horse on a good day (I did do the maths once - I think you could get 90hp from a 1043cc G40, assuming you could source a suitable head gasket?).... or do a few hours overtime and get a 1.3 MPi from a breakers. Maybe with a 5 speed thrown in if yours is currently a 4. Either a wide ratio one for better motorway manners, or a close shift GT spec one to make the best of what is, these days, sufficient but still not a great bundle of power (75hp). Plus, apart from not matching up with the numberplate, it'll be a stock setup and easily supported and diagnosed.
Or a 1.3 genny engine on the cheap, that's still an extra 10hp and almost as much torque as the GT below 3500rpm.
Otherwise... ditch the weight, get as wide a tyre/rim as will sensibly fit, and drop the suspension. Make up for top end speed with better acceleration (maybe even as good as the 1.3 if you chuck out the back seats) and improved speed through the corners.
And yes - NOS! NOS! NOOOOSSSS!!!
I want to see someone do this at last!
EDIT: Jesus, the last post here was last March? What happened to the place? Normally a thread this far down the first page would be but a couple days old