8 Valves??? Lolz. Even my mitsubishi glx 1991 has 16 valves man. That's like 1 valve for intake and 1 valve for exhaust... I guess I'm more inclined on the 90s engineering. Best advice ko is scrap the engine and place something more powerful inside like a 4G63T or 4G63 if it fits that is..

Check also for bearings etc etc. Bad bearings give powerloss. But since engine is so old I think it's best to have it rebuilt if you really love it.