that might be true for newer engine, but K series engine is technology 40 years old (well, nearly 30 years for 4K engine design) and it really isn't made for performance, instead, they just last forever as an inefficient workhorse -- low powered, high fuel consumption engine that just goes on and on without failing (good thing actually if petrol is free and you don't need that much power...)
running on low compression piston and pushrod overhead valve design, it's just not suitable for modification. Yes you can custom made high CR forge pistons and probably source the ever-impossible-to-find-TRD 16v head for K series engine, but you might as well swap engine, more cost effective.
but if mod the stock engine is the only choice, then it can't really be modded like most modern engine's stereotype "lite-tune" path. One thing for sure, for power, try source a high flow carb and feed your engine with all the petrol you can afford wahahaha..