How do I obtain the board/plank that is needed to get across the Lava?
You need to pry them off with a crowbar.
There is a crowbar in a room you haven't been to yet.
Spoiler
Go into the ufo. Look carefully at all the sections of wallpaper. There is a hidden room you have yet to visit.
Ah, thank you very much.
By the way, I enjoy your website and its forum.