It depends on the power goals you have. Roughly the required injector size in cc is the number of hp you want. Like if you want 300hp, then 300cc injectors are minimum. Stock 964 injectors are 220cc IIRC but 964 has higher base pressure than 3bar (it was 3,8 IIRC), that's why it gets away with these minuscule injectors, but it's barely adequate with stock power.
If you look for injectors, avoid ancient EV1-type crap (the ones that look like stock fat-type injectors) because the have poor response and atomization. Prefer newer, pencil-style injectors, they are directly swappable with the old ones.
They look like this:
With properly tuned standalone, modified cars pass smog tests easily assuming the cats are still present and sometimes even without it (depends on lot of things).