Oil Light always on on my ZX50 MOPED

[JADE56]

I BOUGHT A SECOND HAND zx50 Moped. iT ONLY HAD A FEW Km on the clock even though it was 5 yrs old. The Oil warninhg Light is always on.
My usual mechanic said he would possibly need to replace the whole oil tank and could cost $200 or so.
 Someone else said it Might need to ditch all the oil and put Completely new oil in tank. At this point I have done nothing. I just make a point of topping up the tank every week or so.
Also Can someone tell me what is the volume of the oil tank in ML.

[zombie]

It holds ML till full, after that it spills on the dirt.

Inside the oil tank is a float on a shaft. Sometimes that assembly goes bad, and you can get that here...
35410-GR1-9000 SW ASSY OIL LEVEL | is a saleable item
http://www.kymcopartsmonster.com/p/Kymco-Parts