Fuel smell in oil can be normal if you are running an very toxic fuel, fumes leak passed rings way before the fuel itself. My oil smells a little like VP u4.4 whenever I drain it. If you ever smelled that you know how toxic it is. Or if the engine is stopped while fuel is on top of the piston it might slowly leak down the cylinder walls. But it sounds like you’re describing serious fuel amounts in your oil. You could do a leak down test to see if your rings aren’t sealing. Although your diagnosis for fifth gear seems correct, it all has to come apart anyways.
Here’s a link to a reputable members’ sticky in what ever SX should have. https://www.ktmatvhq.com/forum/sx-en...asic-mods.html