1999 Subaru Impreza Engine with noisey tappets.

hi there, i have a 1999 impreza uk turbo 2000, it has got full service history, always used fully/part synthetic oil, now its got fully synthetic oil, its driven hard sometimes, the tappets have gone extremely noisey and i think i can also hear some piston slap, the car is also jerking sometimes when driving, does this mean the engine is on its way out? or is it repairable? thanks.