My son had a 1986 KX80, and I don't think it used a gasket to seal the cases. If I remember, it only required case seal the same as newer models. While it still could be leaking, I would suspect the wet side seal first. I think both the dry side and wet side seals are replacable without splitting the cases, and are relatively cheap, so probably worth a replacement before you split the cases. (I would replace both of them, as a bad dry side seal can lean the bike and lunch the big end rod bearing.)
It also sounds like your needle and seat are not sealing well enough to shut off the fuel, or your float may need some attention. In any event, the entire problem may be as simple as cleaning the carb and changing a few seals.
Good luck
Rick