If you filled the tank and left it for a day, with the plane sitting level, only to find the tank half empty, then I'd say you messed up the tank when you crashed. (Assuming that your engine is mounted upright. If inverted the tank could drain through the carb.)

It sounds as though the tank only drained about half way, which suggests a few possiblities: either the tank has a split about half way down, or the stopper was dislodged, or one or more lines are leaking where they enter the tank.

No matter how you look at it, you're going to have to pull the tank. It SHOULD be removable without cutting any wood, but I don't know what the plane is.

By the way, a bad fuel line could very well be causing the problems with your engine.
Dennis-