Oil in coolant

[Camkdx]

Got a 91 kdx200 and honestly my first 2 stroke ever. So normal issues and solutions are new to me with this as I read and research my issue. I believe it is my water pump seal that's causing the oil in my radiator but then again I haven yet to find a forum with this specific issue posted. Before I go just replacing the water pump seal I want to be sure of all possible causes so I will better prepared since I must order any part I need and the delivery time is 2 weeks or better since there are no spare parts for this bike in Cambodia.

The possible cause of this issue coming about: was out romping yesterday heard a loud pop sound, bike acted as if is wanted to stall out for just a brief moment. I have no power loss what's so ever and the bike still runs like a scolded dog. Coolant is also going into the gear box. I have not drained it yet. But when I checked my water levels this morning there is oil sludge under the radiator cap and quite a bit. Doesn't smell like and gas is mixed with it neither.

Story of the bike. It has been sitting not ran for over a year other than just a simple crank up. The guy I bought it off of was terrified of the bike and also just an importer.

[Jaguar]

yeah, replace that seal that separates the coolant from the transmission oil.
When you said it "popped" I immediately thought of a blown head gasket. But that wouldn't allow coolant into the trans oil. But being an old bike you should order a head gasket also because it could be both things at once.