Yes, your right, it's not an ATV or go-kart but the oil seal for the oil pump on a Manco ATV is the exact same as the one on the NSR (9.5*16*4).
Unfortunetly this part is now obsolete but Winderosa made/makes an OEM replacement part which I'm trying to find out if it is still available.
Have a look at the following thread, according to this it's a direct fit.

http://nsr250.net/forums/viewtopic.php?t=11190&highlight=oil+pump