I have not worked with an abused horse, but of all the horses I have worked with, I have found that trust happens quickly.
The key in my mind, is to be consistent and fair. The first sign of a bite or kick, gets a sudden 'hey' in the moment, second sign or a progression in their aggression gets big movements from me and a volume to my words. The third stage, I whallop them. I would not care if they have been abused or not, it is not ok to come at me, I am not abusing them.
To me and having worked with biters and kickers (and abused dogs and kids not horses ). But it does not matter the abuse, and fear of humans is the same species wide, you have to be fair and consisitent, you cannot change the playing tables.
If you are always fair and consistent your horse will learn in no time what you expect.