Pistol grips are nonsporting. So yes, you would be in violation. Depending on the model of SKS, you have to then begin replacing parts (and if a Yugo 59/66, you also need to be removing a few other items too; there's a wealth of info on it out there).
If you plan to take an SKS and mod it into a cheaper AK, just get an AK- you'll be spending just as much by the time you finish and you may not retain the reliability (you'll have slightly better accuracy, but not by leaps and bounds).