Try drainin all the oil out of it then put the plug back in and see. That'd probably tell you whether it's tranny oil or from the premix. There might be an extra shift shaft seal out there, not sure.
Someone else should probably mention once you getting into this cub vs. stocker it's not just cylinders you're talking here. Lock-up, cover, override, swingarm, tires, etc. etc. is all needed too.
But as for the cylinder choice I would go cub.
Describe the problem a little better. You make it sound like it's hardly running by your 1st post, but then said it's just a little boggy or something?
Same carbs, around 1100 ft. elevation and hot riding temps. I'm at 155 main and 48 pilot right now but I think I need to go up a little on the main I've got a little bog at WOT in the upper gears. Haven't had much time to mess with it though but you sound close.