As far as I know there are no replacement rods available......Tuck had some made...see here
http://edetuckracing.blogspot.co.nz/2011/12/i-got-package-in-mail-today-x-nsr250.html

Some Rod and MC18 crank rebuild discussion here
http://www.nsr250.net/forums/viewtopic.php?t=7806&highlight=