OK, I checked on Kawasaki's website, the specs are not listed before 96, but here are the 96 KX250 Specs for the transmission:
Transmission
Clutch Type Wet, Multi-disc, Manual
Transmission 5-speed, Constant mesh, Return shift
Shift Pattern (Bottom up) 1-N-2-3-4-5
Gear Ratios
—1st 2.13 (32/15)
—2nd 1.69 (27/16)
—3rd 1.39 (25/18)
—4th 1.14 (25/22)
—5th 1.00 (24/24)
Primary Ratio 2.75 (55/20)
Final Ratio 3.50 (49/14)
Overall Ratio (Top Gear) 9.62
gear ratio*primary ratio*final ratio = overall ratio for each gear.
Overall ratios
1st - 20.49
2nd - 16.26
3rd - 13.37
4th - 10.97
5th - 9.62
With that info we can figure out (3.5*20.49/16.26) that we need a final drive ratio of 4.41 to make the move you want. If you do the math (rear sprocket/front sprocket = 4.41) you can plug in the various front spocket sizes to come up with the rear sprocket.
Hear are the answers:
14/62
13/57
12/53
Now, you may wonder why I didn't just give you the answers. I hope by explaining it you will understand how to figure it out next time. The only viable option is going to be the 12/53, so you probably should compromise and go with something like a 13/52. You should be able to now figure out what kind of ratio that will give you.
Good luck.