As LG has pointed to - forget straight oils.
Use a penetrating corrosive lubricant, WD40 (TM) is
one such off the shelf at hardware stores.
At the bare miniumum you need to remove the head.
Wet down piston top and valves with head upside down.
When attempting to "free" any components use only
copper drifts - many persistant taps with a light hammer.
Hardwood timber tapped "on the grain" may give similar
success.
Where the unit construction will not allow covers to come off
or you cannot get any joy with the above AND you really want
the engine, put it in a sack and throw it over the back of a dinghy
on short rope. Row around the ocean for an hour or two and retrieve.
It will come apart then, using the tap an' go method.
FWIW - combustion engines left to moisture intrusion are usually
beyond repair after a week exposed to air, wet.
Trust that helps some.
KK