To an extent, the old saying of 'There's no replacement for displacement' does have a string of truth to it mate, you'll always be better off getting a car with more power in the first place, so a step up to the thirsty bazzer that is the 156 V6 might be a decent idea?
Prices are bottoming out now at about a grand for decent enough examples on early 98/99 plates, it's alot of car for the money and comes ready with 190BHP, it doesn't take much to get them upto GTA spec either with air intakes, exhaust upgrades, chips etc etc...
Always remember though that you can't give a car that's built to withstand 144BHP a whacking great 250BHP and expect the brakes and everything else to be able to withstand it.
Once you start dishing out big power to smaller engined cars you gotta upgrade everything else.
For BPU's (Basic Performance Upgrades) though you won't have to, exhaust, intake, and chipping a car will give you around a further 30-50BHP I reckon, and you shouldn't have to upgrade anything else.
That lot will cost you around £700, possibly more for EXTREMELY decent gear. But red.dot to remaps for about £200, but that gives around 20BHP extra I've been told???? So, your 1.8 Twinny with 140 Something BHP will have around 160 Something BHP. Then a new exhaust system, possibly decatted although illegal, will give a fair bit more, as will the air intake upgrade...
You can reach around 200BHP from a twinny without serious issues mate.
Good luck with it and keep us posted,