you have already accomplished the most difficult part of the mak-90 gas piston swap...locating the retaining pin.

the pin may be thru and it may be blind. a few good blows with a hammer and drift punch will indicate if the other side has a visible retainer pin head showing. after polishing the carrier/retainer pin heads to match it can be very hard to even find where the pin enters and exits the carrier.

if the pin is thru, simply drive it out and unscrew the gas piston from the carrier. if the in is blind, center punch it, drill it below the carrier wall thicknes and unscrew the piston.