Stopped at HomeDepot to get a BFW, an 18" Crescent wrench , as my existing 12" adjustable wrench and 18" pipe wrench have not been able to remove the engine exhaust gate valve. The BFW combined with some PB Blaster and gentle persuasion of a rubber mallet applied to the wrench did the job nicely. The interior of the gate valve was rather sooty from the engine exhaust, but the gate and the slot into which the gate fits to close the passage were not corroded or deformed. The packing nut and stem showed a good quantity of green copper. Still the valve opens and closes easily. Nonetheless, it will be replaced with a 1-1/4" in-line ball valve .

Engine exhaust valve
Engine exhaust valve interior