You're substituting one difficult to find Cub accessory for another. PTO reverser/reducers are just as hard to find as Howard Rotovators.
You will still be left with the problem of the Cub wanting to go 2.3MPH with the throttle set high enough for the engine to produce enough power to run a rototiller. That is much too fast unless you're tilling loose silty loam. So not only would you need the PTO converter, you will need equally hard-to-find speed reducers to get the Cub down to creeper speed. The speed reducer is part of the whole Howard Rotovator experience.
DR and AgriFab both currently make tow-behind self-powered tillers, and there have been countless others sold over the years. Even then with the tractor at a bare idle in 1st, ground speed still might be too fast.