Assuming everything is in good tune
Then Newman Cam (solid profile and lifters)
Decent air filter (eg K&N)
Headwork if you can afford it/take it off the road for a bit (or buy a head thats already done)
You could get a chip also - tho id do this last if you are doing other mods, otherwise it will need a new chip again at a later date
You may also need injector upgrades depending how far you go

Most people will come along and just say replace it with an RS Turbo engine tho