Aldo, it would have been worthwhile inspecting the valve seating while you had the head off. I agree that head gasket was leaking, by the way. You should also check the tappet clearances.
Some of that looks like sediment in the float bowl, and some of it looks like corrosion. The trick is to clean it up without removing more of the interior surface finish than necessary.
I haven't seen a mistake in the picture. Incidentally I think the in-line fuel filter may be a good idea - I'd rather keep the fuel clean, but you can't always do that in practice. You have the red filter, which is correct for gravity feed.
