No one ever said Altezzas where fast Although once you get them wound up over 5K they go alright.
Basic mods to start with after the air filter would be some Iridium spark plugs (NGK or Denso). Upgrade the earthing system. Clean the MAF sensor & throttle body. Replace the gear box oil with some RedLine MT-90 & diff oil with some 75w90 or 80w140 fully synthetic. Put some fully synthetic 5w30 or similar grade in the engine. Basically after doing all this it'll make it run like new again & maybe gain you a few HP.
If you want to get 240bhp out of it, you'd be looking at replacing the ECU with something like an Apexi Power FC, some mild cams, headers & possibly ITBs. Then swap the OE dual mass flywheel for a solid lightweight aftermarket unit from JUN or TRD.
You could go silly with a stroker kit but you'd be better of spending the same money on turboing it IMO.
Unfortunately there's no cheap 'n easy power upgrades for these things.
As for sourcing parts there's plenty of places on the net that'll ship direct from Japan like Nengun, JPS & ACNZA
TezzaWorld & Altezza Club NZ are two good forums you should have a look at to get some ideas.
Don't forget to upgrading the suspension as it's well worth doing. Then you can enjoy the increadable chassis these things have to the fullest.