Welcome to the “family”.
What you see going up and down below the spark plug holes are actually the valves, not the pistons. But that’s a good thing too. Sometimes the valves get stuck open after sitting for long periods.
On the oil filter, give the cover some taps around the edge with a small hammer, or rubber mallet. The gasket is probably gluing the cover to the canister.
On the air filter, spray down the wing nut with some PB Blaster, or similar, and let it soak a bit. It’ll probably come off OK. Paper filter conversions aren’t too difficult. I did mine many years ago, but mostly because it had been sitting in the weather when I got it and parts of it were rusted away. Otherwise the oil bath, when cleaned and serviced properly, is a very good air filter.
It looks like it’s had a few different layers of paint, and someone has been stripping some of it off already.