Farmall Cub

Does anyone have tips on replacing reverse gear

I would make sure the replacement gear has a new bushing installed in it. Other than that if there was any gear to replace you have picked the easiest one.

What is the problem with your reverse gear? Chipped tooth, noisy, etc.?

Are you talking the the lower reverse gear, the reverse idler, or the 1st/Reverse slider?
In my opinion, changing any one would require changing of all three of these plus the lower first gear due to them wearing all together.

Replacing the reberse slider, it is missing a tooth and makes noise

You have to split the tractor at the transmission to slide the main shaft forward to allow you to change the slider gear. This opens a can of worms in terms of "while you are there, you may as well ..........."

Splitting the tractor from Rudi's Manual Server:
http://www.cleancomputes.com/Cub/Blue%20Ribbon%20Service%20Manuals/GSS-1411%20Service%20Manual%20for%20Cubs%20and%20Lo-boy%20Tractors/Section%204%20-%20Splitting%20And%20Recoupling%20The%20Tractor.htm

Transmission from Rudi's Manual Server:
http://www.cleancomputes.com/Cub/Blue%20Ribbon%20Service%20Manuals/GSS-1411%20Service%20Manual%20for%20Cubs%20and%20Lo-boy%20Tractors/Section%206%20-%20Transmission%20And%20Differential.htm