The biggest so-called legal engine mod you can do here is, to
swap your current one with a similiar unit, of the same capacity
and model code, sourced from other countries or from a deregistered
vehicle.
Not sure if this applies to same type of engine, but of different
generation, ie a B16A from a EG9 Civic for a B16B from a EK9
Type-R. I mean from the outside, these 2 engines look the same or
can be done to make it look the same... play around with the papers
and voila. 
