I don't think the timing would be a problem, if you're the first one to open it up. I would hazard a guess that you are!
I used a soft (brass, I think) wire cup brush on a drill to clean up the head. You can use the same method on the piston and valves if you like, or you can use a plastic scraper or nylon brush. DON'T go too hard around where the valves sit! If you change the shape you'll need to reseal them. Make sure the piston is at the top of its travel and the valves are closed before you start cleaning. Lightly blast any loose stuff away with a bit of air directed across the assembly, rather than straight down at it. There are also solvents you can buy to help you out. But most of that looks like it would come off without too much effort.
Sounds like you had a stuck valve. It is still a bit tricky to tell without being there in person, but they look like they are functioning normally in your video.
