Before the jokers rush in and tell you to change K&N air filter, put cold air intake, change exhaust system. I'll tell you don't waste your time. Its not going to be worth anything given the weight of the W210. Curb weight 1.7 tonnes. On paper 134 bhp and 190nM (19.35 kg-M) torque, and over the years some horses died.
Toyota Vios Turbo: Killing Boredom
Here's a dyno of a W210 with the 2.4L V6 Engine, 137bhp and 17.89kg-M torque, compared to 168 bhp and 225nM (22.95kg-M) when new. So dead horses and tranny loss is about ~20%
Using ~20% as tranny loss & dead horses correction, your car would make about 107.2bhp & 15.48kg-M on wheels. Keeping in mind the car is 1.7 tonnes, even at 137 bhp, its a waste of time. To tune your car to 137 bhp is an increase of 27.8%. Natural aspiration tuning to gain 27% is
very expensive. And even then, its still slow -- remember, the car is heavy!
For comparison sake
Power to weight ratio (higher is better, more hp per lbs)
Your W210: 0.0283528292
2.4L V6 W210 in the dyno: 0.0362344925
Myvi 1.3L: 0.0319632958
Dyno for Myvi:
http://www.strombergpowerbooster.com/ifc/images/myvispecvonwheel.jpg
Proton Gen2 CPS: 0.0405282939
Dyno for Gen2 CPS:
http://images.paultan.org/images2/Proton_GEN2_Campro_CPS_Dyno_Chart_Large.jpg
Even IF you successfully modify your 2L to the performance levels of the 2.4L V6 engine, it is still pathetic and waste of time, and the makcik in the Gen2 CPS will have more power from standstill climbing the slope and you'll still look like a snail.
If you want it to be significant to make a difference and not "underpowered". Slap a turbo or supercharger on it, or swap in one of the AMG mills. Otherwise, don't waste time, get the new CGI or CDI Mercedes.. it'll change your perception about power.