A good penetrating lubricant, some soak-time, make sure the captive cotter is loose and can be moved about with your fingers, then a series of light taps with a hammer on the angled part of the clutch, rotating the engine about a quarter turn between taps. If it doesn't move at all after you've done that continuously for about five minutes, it will be time to take stronger measures. That means borrow a 6 inch gear puller, or get a local motor mechanic who already has one, to pull it off for you.
The right engine oil for that engine is ordinary automotive engine oil, but grade SAE30 (not multigrade). However for the compression test, you can use multigrade if you happen to have some.
