OS FS40 stuck exhaust valve

[hohocc]

I'm in the process of freeing up my very old and gummed up OS 40FS, it's got an exhaust valve which is stuck open, appears the problem lays with the pushrod/tappet as the rocker arm does not move when the engine is turned. As a first line of assault I've filled the top of the head around the rockers and valve springs with CRC to see if it might soak in and help a bit. Tried to remove the cam cover plate but while I can turn it easily it seems to need some force applied to get it to pop out which I'm reluctant to do.
Other ideas I've had are warming the tappet area with a heat gun or (carefully) with a butane torch, or boiling the whole engine in water (as worked well with the carb) and then running it immediately after to make sure it dries out. Neither of these ideas really appeal so I'm after any ideas!
Thanks.

[AshleyB32]

if you used castor oil in the engine previously boil it in water and get it apart if your worried about being able to dry it after you can always heat the oven up to like 150 and turn it off chuck the engine parts in there and let it dry out, ive done it b4 with model engines 2 & 3 stroke, give it a good oil after your finished and you should be good to go