Most likely the exhaust valve is stuck open from sitting. I ran into similar situations with both a Super H and a 300 that sat unstarted for several years. This fix worked for me:
First spray the stuck valve(s) with PB Blaster or similar. Now to remove the rocker shaft/rocker assembly to get at the stuck valve's stem. Gently hit the end of the valve with a brass hammer, a steel hammer with a brass drift, etc. Unless the valve is stuck really hard, once the valve is driven in 1/8" so it will suddenly spring free and return to the closed position. Repeat the spraying lubing and tapping until the valve closes easily with spring pressure.
If above doesn't do it, remove the spring keeper, cup and valve spring from the stuck valve. Then apply GENTLE heat (low propane torch flame) to valve stem near the guide. Tap the stem a few times with your hammer then spray the stem with PB to cool/lube it. Finally repeat the the tapping exercise above.
Once the valve works freely, check all the pushrods to make sure none are bent - any bent push rods must be straightened or replaced. Then replace the rocker arm assembly making sure all 8 push rods are engaged with their rockers. Finally check/adjust the valve clearances.
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There's plenty of clearance inside the cylinder for the valve. There's no danger of bending a valve as long as you don't get too enthusiastic hammering on it.
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