Here are a few more ideas for 223 but may also apply to others.
Air filter, keep the original housing but install a K&N E-2725 (for a Mitsubishi Monteiro I think) fits perfectly.
Add a few horse power. Ditch the steel fan. With the 223 there is not enough room for a single fan, so use two 10" fans mounted on the inside. I made a frame to mount them on rather than simply "wire tying" them to the core. The water pump pulley falls nicely between them.
If you are into the motor, balance moving parts. My crank needed 80 grams removed from one counterweight to get balanced and the connecting rods varied 40 grams (40 grams!!!) heaviest to lightest.
Camshaft. I searched and watched for a year for an improved performance camshaft and did not did not see one old "Isky" or Crane cam (maybe they never made one??). Sent original cam to American Custom Cam in Valencia CA, they did an amazing job on the cam regrinding to a new profile with a nice idle and improved mid range, also refaced the lifters for under $200 including shipping.
Good luck with your projects.