There really is not any "pep" left in that engine, unfortunately. With no turbo to tweak, it is basically only going to benefit from a "tune" to the timing curve, which is worthless since all that does it force the full advance which you can do yourself by putting premium fuel in. That'll crank it as far as it can go before the knock sensor signals the ECU to cut back. And even then, you are talking a very small amount of difference. That engine does not specifically require premium gas.
There is a reason those engines have been given the moniker "two-point-slow". But, they still are a very good engine, and sometimes finishing the race is more important than winning it.
Here are some tips, though:
in 2003, there were two different 2.0L engines, the AVH and the AZG. They are mostly the same, but there are a few things sometimes different so you should be aware of which engine you have before ordering any parts. There are other SOHC non-turbo 2.0L engines, too. AEG (older) and BEV (newer) for example. So again, important to know which engine. Dealer can tell you by VIN if you cannot read the white tag on the upper timing cover or the white tag in the wheel well in the trunk.
These engines *can* use some oil. Until you know for sure what type of appetite your car has, check it frequently. It requires a 502.00 spec 5w40 oil (most any 505.00 or 505.01 spec 5w40 diesel oil will also meet this spec). 10k mile intervals, same as the TDI. I would use a Mann, Mahle, or Hengst filter.
They are geared much lower than the TDIs, so you will find a bit different shift pattern. However, for a gasser it is still a reasonably torquey engine with a pretty flat power delivery. Not an angry chainsaw engine like in a Corolla or Civic or Cruise.
