If I were going to make one, I would get a 1/2" bolt, 2" long, turn down the end to 3/8" for 1/2" long at the end, cut a 3/8" UNF thread on the 1/2" end, and use a penny washer to make it wider at the top.
I would also smear the thread with copper-ease to stop it rusting in the chassis again. But the reaction lever is seldom removed, normally only the take the body off the chassis, or to change from LHD to RHD or vice-versa.