If it was me, I'd probably start by removing the barrel, then putting a thin parallel punch through the transfer port and tapping it gently to see if the piston head is going to move. If it does move, you're on your way.
Bearing in mind that the port is probably not central, there is a chance that, if the piston head moves, it may tip and jam. It might be worth putting the piston down the cylinder in reverse to hold the piston head square to the cylinder bore.
If you can't face removing the barrel, then it's a very long thin punch you'll need.