I came across this video on Youtube showing some chaps fitting a replacement spring on a CRF250 Rally. I’ve no idea where the spring can be purchased from or if it was a custom one off manufacture.
The good thing about the video is that it shows how to take the OEM shock apart to fit the new spring.
Once the spring is fitted it appears that the sag is almost non-existent and the rear of the bike is much taller even with rider aboard. There’s very little info in the video description and many have asked for more info but nothing seems to be forth coming.