Tricky, there are no water galleries in the bottom end. There is a pipe brings water in from the rad, then another taking it to the cylinder.
The water stays in the barrel and head. Being a 2stroke there is no valve trrain to complicate things or allow water in either.
You need to check the pump fully, there may be a crack in the casting allowing water to pass, maybe a wear ridge on the shaft.
Could you remove the rh case and then do a pressure test much like a bike puncture. The system only runs at about 6psi so a couple of corks and a football pump would do it.
Does seem odd though.