According to Wiseco.com 78-07 KX250 all used the same rod kit which retails for 91.91:
Wiseco Part#: WPR133
Product Type: Connecting Rods
Description: Rod Kit - Kaw. KX250 '78-07
They do not have a complete crank not does anyone else that I could find.
According to Kawasaki the 89 KX250 crank is year specific: FITS 89 KX250 - P/N 13031-1292
That is not to say that another year won't fit, this is Kawasaki's info.
Perhaps another member has had experience with interchanging other years....
Then again, if your crank halves are good then I would get a rod kit and have it rebuilt.