Headwork, cam and laguna 1.8 SPI Throttle body would give a GOOD gain.
You could also slap a 1.6 8v bottom end from a megane on...its a direct fit, should add more torque.
With all that you'd probably see 100bhp or therabouts. For more you'll need a different engine...1.8 16v, 2ltr 16v, 1.7t, 1.4t...don't think anything else is really worthwhile, unless you've a strong attachment to the rest of the car.