Guess you probably need to start with asking why 20psi? Is it just a cool number, is there something you expect at 20psi?
If you can settle for 15psi and 18ish on occasion, there’s no reason to open the motor. Find yourself a good tuner and follow one of the few 4g9x turbo recipes on here.
To handle 20psi all day every day, you will need forged rods and pistons. K1 rods are usually the go, and pistons can be sourced from several places. You will of course need new hardware, tougher rod bolts, main cap bolts and head studs. ARP stuff is all through my motor.
Replace the oil pump and bypass. A new pickup wouldnt go astray either.
The ideal setup is to find a CM5A dohc head, but they are rare and pricey.
Plenty of re-grind cams around for the sohc, just chase one with the specs to suit your desired power band. Std sohc head won’t hold you back if your goal is 20psi - it may hold you back if you want x kw.