I would either sell up and get a 2.0l, or accept it will never be a rocket ship, maybe lower it to improve handling in the twisties and look at keeping it well serviced and check on stuff like bushes etc to keep it tight handling, then maybe a decent exhaust and filter to help breathing.

Not really worth chucking cash at it as it will never be quick and will be £££££ to get any big improvement, big car, small lump = slow.