I'd apply some penetrating oil to each valve, let them sit awhile, and then try twisting them loose. If unable to get the valves loose, I'd just pull the valve and the seat both out of the pump. If I remember correctly, the only thing holding each seat in place is the nylon seal ring.
If the cone valves are badly corroded or you damage them getting them loose, you might want to consider replacing all of them with those ball-valves in your other pump. Not sure if you ever had a problem with hot-starts, but when these cone (check) valves wear out, the metal fuel lines will leak-down and make hot-starts difficult.