As BDG says tuning any 2v lump over 100 horses isn't cheap or easy. There's lots you can do to change the character of the engine, big bore for more torque, lightened flywheel for better throttle response / more revs, but in the long run you're probably best with a chunky zorst, open airbox, k&n filter and a bit of dyno time.
The next step is a pair of testarossa heads, but you're talking £thousands....
Enjoy the numptystroodle, and welcome to the site, I'd love a pint of guinness.
Cheers,
Ali