You can get any sr20de parts from autozone, Nissan, pepboys etc. But either way, if you want to stick to the KA, simple mods I can think of for the ka is removing the clutch fan and going with an electric fan. It made a pretty good improvement when I had a ka. Back in the day, I entertained doing what you are doing and going all out na ka. With a crank pulley, removed clutch fan, 3" catback, and intake and no cat, it made 154whp, and 149torque. Going with high cr, cams, and intake manifold you'll probably get around 175whp. It would cost $$$ though. One thing you could do for cheap is supercharge it. I supercharged a sentra for $250 including a mercedes supercharger, piping, bolts and gromets, rb25 side mount and a supra bypass valve.
Cheap as crap, make 200+ for the cost of an exhaust. If you want 250-300hp, you would have to drop another $700-1000 for a safc, injectors, and clutch etc.. So it just depends how much power you want. If you are happy with 200whp, then look into supercharging it for under $500 with stock fuel, stock computer and clutch, but after that point stock crap wont be able to withstand more power and its the line where you will need a good $1000+ to proceed. If you want a 200whp NA 4 banger, and dont want to spend rb26 prices, then look into a sr20de/ve hybrid. If your happy with 160-170whp, then stick to the na ka.