Can you upgrade the plastic 89 KX 250 with a body kit and seat to a more modern look?

yes you can, kx500s were the same from 88-04 and in 89 the kx250 and kx500 were the same ecept for the motor, you can basically upgrade your 89 kx250 to an 04 kx500, including the newer forks, tank(which is bigger), swing arm, fenders, seat, etc