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? 

My recommendations since Kensei abilities really don't add much even originally. They need the following abilities distributed over their subclass.

1. Dedicated weapon - 2, possibly a 3rd later. Remove weapon restrication as the monk die is larger now and makes it irrelevant. Further there is plenty of fantsy concepts where Kensei monks dedicate their combat to style to exotic weapons. Giant war clubs, to steel fans, to chain and sickles not just samurai knock offs.

2. Weapon Mastery of your Kensei weapon. I mean it's literaly the whole point of the subclass.

3. Empowered Strikes affects your Kensei dedicated weapons

4. Kensei Shot redesign : Once per turn you can add an additional roll of your martial arts die as force damage when you successfully make a hit with a kensei weapon. This brings it directly in line with similar monk powers. Also would make crits feel a little more exciting. No action.

5. Deft Strike: You may substitute any unarmed strike with a Kensei weapon attack. ie your bonus action and flurry of blows. Again most likely your damage die from unarmed strikes will already be as high or within one die of damage. We're talking maybe +1 damage.

6. Sharpen the blade redeisgn: Expend a ki point and gain a fighting style that uses your Kensei weapon. This effect lasts for 10 minutes and ends early if you are knocked unconscious,

Keep the rest. Basically your monk that doesn't need to take Fighter dips for weapon mastery and fighting styles. Which really strikes me odd that monks get neither.

How do you find the Xanathar's Kensei implementation in D&D Beyond nowadays? As far as I can tell most of features don't work: you don't get weapon mastery for the kensei weapon nor it is treated as a monk weapon, equiping armor prevents Unarmormed Movement and Martial Arts (which it shouldn't since lvl 6). There may be others I dind't notice but these two bother me.

Do you have any experience with workarounds or maybe a Homebrew Kensei that works well in 2024 (and someone did the work that WoTC should have done)