Can't get the chain door on the motor open!

Try this: Use a screwdiver of the proper size and hold it firmly into the screw head then give it a good whack with a hammer. It might loosen the screws for you. Just a little heat from a propane torch might help too. Don't cook the engine case just warm it up. The aluminum will expand quicker than the steel screw and will help loosen the grip.
It's important to use the right screwdriver however. It must fit the screw head.

As for feeding the chain into the sprocket, you might have luck by tying a string to the end link and if you get the string through, pull it as you turn the sprocket with a screwdriver. What happens is the first link to pass over the sprocket will bind inside the case and not follow the sprocket teeth. If you get it passed that point it will go easy from there.
By the way, that "door" is called the clutch actuator cover. It contains the cam which operates the clutch mechanism.

Good luck, let us know how you do.
Tom