I measured from the mount on frame to the fork leg with bars turned all the way to the right. Then I measured the same points with it to the left. I took the difference to get the stroke.
Harris peformance makes the bracket for the fork. https://www.harris-performance.com/website/frameset2.htm
Search for fork leg clamp. The bracket that holds the damper should be included.
I noticed that people mount them different. Some people mount the eye on the damper to the frame and others mount the eye on the fork bracket.
I should have mine on Monday, will let you know if the 120mm is a good size & how I mounted it.