basic rebuild ... possible?

You won't be able to see the skirts until you pull the pistons out. They are 90 from the piston pins. To make a long story short, the skirt is the side of the piston that rides on the cylinder wall. Take a fine piece of sandpaper and remove the carbon at the top of the cylinder. See if you can catch a fingernail on the ridge. If you can't or just barely can, you can get away with a hone job. Don't expect a lot from this engine when your done though.

Your cam looks OK from what I can see. Check the top of every lobe and see if there is pitting or galling.

Is that rust in the intake ports? You should replace your pushrods, they are truly nasty looking. Stock ones are pretty cheap.