I was adapting one of my Kensei Monks to 5.5. I played the UA version, which also had an extra 1d4 bludgeoning attack as a bonus action, and "Precise Strike".
When adapting, I was only able to add Mastery with my Kensei weapons, which I think makes perfect sense, it's the minimum a Kensei Monk should have.
To try to balance it out, remove these two features from the UA version of the Kensei Monk.
Do you think it's balanced?
(Written with Google Translator)

Adding weapon mastery to kensei monk makes perfect sense.

Spending a bonus action to get 1d4 damage is not great.  If you have focus points, flurry of blows is way better.  If you dont have focus points, one bonus action punch is also better.  If you want to keep it go ahead but its a bad feature. 

If you are allowed to use tashas optional rules for monks, i like deft strike over precise strike.   You can spend 1 focus point to add 1 martial arts die of damage, (which is ok i guess, but not great, ) but that triggers ki fueled attack to make a bonus action attack with your weapon.  This works very well with a lonbow build.  Other wise nick mastery and dual wielder feat are excellent for a melee kensei monk.  

For the 6th level feature letting kensei weapons deal force damage would be the updated version of magic damage.

I agree, the d4 Bonus action attack is not worthwhile. And Precise Strike isn’t that great either. I don’t know if it is necessary to get rid of those because you are adding Weapon Mastery. I would just keep them even if you don’t use them. I don’t think it will be a balance issue keeping them. 

Any reason you are not using the official release of the Kensei?