Probably the countershaft seal leaking,common fix and easy to do.
New bushing/o-ring/seal and its good to go.
A bigger tank from a 2000 era 250/300/200/125 should fit,KTM is very modular and many parts fit on many different models.
I had a new 2001 200MXC and thrashed it for 4 years to no avail,couldnt break it.
Some bikes around at times