Hey Mark, we are assuming that the cause of the problem is the O ring, not metal seizure and have no idea as to the equipment you have at your disposal or your knowledge of models or engineering. The dowel will take the shock, though that is what you need so the use of wood or rubber mallet will not help, a normal hammer is fine. Place the cylinder on a solid heavy object,try to protect the machined face with thin wood or Etc the idea being to apply the load to the piston whilst supporting the cylinder.
Small press = G clamp ! Say 4 – 6″ Cut your dowel only 1/4 or1/2″ to protrude from the cylinder, keep the cut square at both ends. Remove the cylinder end cover studs if used. Place a piece of thin wood or stiff card on the fixed end of the clamp, may be hold the clamp in a vice. Now hold the cylinder and dowel end with dowel to the screw and tighten ! This arrangement if it will still not see things your way can now be heated. Talk of “dull red” is unnecessary and way to hot ! The O ring will give up long before that temperature is reached. Some one mentioned avoiding the fumes as the O ring burns out – a good plan, best do it outside ! Good luck. Noel.