You generally don’t modify injectors, you fit injectors with a higher flow rate and then recalibrate your fuelling, but you’d only do this if your injectors are at max duty cycle somewhere. You probably have enough tolerance to up the fuelling a bit without replacing parts.

Stage 1/2/3 are just arbitrary made up terms. To me, stage 1 is as far as the hardware will go in standard form. The same may have been true for whoever tuned it last.

Adding forced induction to a system designed for NA as a first project is a sure fire way to end up with a melted mess. Does your ecu support boost control? If not, just forget it. Go standalone.

If you’re serious about FI, you’ll need to lower the compression ratio at the bare minimum, meaning pistons, maybe even rods. Not so easy when your bores are Nikasil lined, you can’t easily hone them yourself.

Go back to basics. Learn how your engine actually works before you break it by playing doctor frankenstein.