Welcome to the forums!
I have done a few LED bulb installs on my bikes, and actually I am also trying to get an LED into my 300 sometime this season. I've got the bulb already, but I've been lazy...
The Ninja 300 has those little rubber cover gaskets like most bikes, except the difference here I've found that makes it harder is that the plug for the bulb actually is run through the rubber cover. Like they cut a tiny hole in it and shoved the wires through, then crimped on the bulb plug.
This is normally no big deal for regular bulbs, but with most LED bulbs, there is a driver box or other accessory that manages the voltage going to the diode. And of course, you can't fit that box into the headlight assembly, and the plug is inside the rubber seal.... Welcome to the most common fitment issue with LED bulbs on bikes, the rubber seal not agreeing with the LED bulb or driver box in some way.
There are a few bulbs with the driver box 'built in' to the bulb, but most people online have said that these are unideal, since the heat from the bulb is now heating the driver, and often times leads to poor performance and failure. Most LED bulbs also have a heatsink, with or without an active cooling element like a fan. The bulb is designed not to be sealed behind a rubber cover, it needs air to cool properly.
For example, my 2004 GSX-R has the older style rubber boot for headlights, and I was able to trim the boot a tiny bit to get perfect fitment. In that case though, the back cooling end of the bulb is able to hang out in ambient air (and rain and all that goodness) and stay cool like it was designed.
I think in your/our case, it may be best to actually purchase a whole headlight assembly with built-in LEDs already as headlights. They are pricey, but they are simpler of an install. My solution to these fitment issues is to design and 3D print a rubber cover that fits the LED bulb, but if you don't have experience with CAD and 3D printing and own a printer and have the right materials, that's not a practical solution for most.
Hope this helps.