if it is not smoking and not reducing oil get a compression test done. if the compression test is fine no need to change the engine at all. stick with the original one.
Any good EFI mechanic can do this tuning for you. a 4g13 is quite a rugged engine and works for miles if properly maintained. there is nothing particularly different in mitsubishi. Also check all the vaccum hoses.
if you have just bought the car you should always change all the fluids and belts and do the necessary tuning.
i dont understand why people have a stigma against a Mitsubishi it is a perfect normal car with normal engine and maintenance.
When ever you visit your mechanic ask him to check all the vaccum hoses, all the belts, timing and tappet clearance (around 10mm). Also tell him to hook up a scanner and check for any code and check the ignition system.