250cc engine is there any possible way of having a 250cc in my ktm sx 125?

Carburetor is dirty, idle circuit needs service.

Your cooling system leaks into the engine. You need to check the water pump for damaged oil/water separator and cylinder's head gasket next.

Thank you for contacting FixYa with your inquiry.
The following link will take you to a good source for your parts requirement:
http://cyclehuttktm.com/FicheFinderNew/FicheFinder.aspx
#1 52330000044 Superceeded Engine Case Cpl.w.tran.bear Look for Part Numbers 31007107001 50330000050.50330022000.503330 Another source: http://www.bikebandit.com/sem/ktm-parts?WT.mc_id=2676712&WT.srch=1
Or Motorcycle superstore: http://www.motorcycle-superstore.com/2/10/213/19924/PDEPT/Dirt-Bike-Motocross-Engine-Parts-2001-KTM-125-SX-Parts.aspx
Sometimes these people will be parting out a 125 SX: http://www.powersportswarehouse.com/fiche_select1.asp?cat=Motorcycles&mfg=KTM&gclid=CPb2ka7s0aoCFYjsKgodZQoh0Q
Hopes this helps you locate a case half. It's a pretty common bike and popular bike. There are a bunch of them out there, mostly in service.
Kind regards,
TF

should say 0.9l on the side of the engine case.

5 litres of petrol 90ml of 2 stroke oil
the best for engine and high performance

The float is stuck open. This causes the overflow and floods the engine.

I would think the it is all one unit, motor oil lubricates that as well.

you probably blew a gasket, any engine is repairable and rebuild-able but if you want the easy and quick way.... go to a salvage yard.