Found a simple way to get it:

Open chapter 14-01 and transport to the desert camp. Talk to Emil (no need to buy anything, just talk, open his shop and exit.This will ensure the doors to his lair will open.) Then transport from the desert camp to the resistance camp (since it's the closest to the pipe that leads to Emil's lair imo).

Run to the pipe rom the city ruins that leads to Emil's lair and the chest in his lair will have the power up L. Make sure to save at the earliest convenience after, otherwise it won't stay in your inventory (I ran back near the resistance camp to save). Then I used the access point at the resistance camp to chapter select 14-01 again and repeated until I had as many as I needed.

Side note, that damn amusement Park bunny never gave me a power up L even on my FIRST defeat. Stuff. That.

In my case, i needed not the Powerup Parts L, i needed the Powerup Parts G. O_o

Can confirm that Rabbit drops Part L. I'm using drop rate +90% (it's the cap anyways). First two tries I did it with no drop rate chips, did not get it. The next three runs with drop rate chips all gave Part L. Ofc, your results may vary but at least, you can farm it there.

The Bunny did not drop it for me even with drop rate +70% and four tries. Neither was it to be found at Emil's place. I could only get them in one particular order: 1. In the Tower after Ko-Shi Now use it to upgrade a pod. Because another might only spawn when there are no powerup part L in the inventory. 2. The Bunny Again upgrade a pod with it. 3. Emil's place It would only spawn after doing the first two. And after defeating Emil I had to go back to the surface, then back into the cave to make the chest spawn.

I can confirm the rabbit's a 100% drop rate (with an extra 100% [two 50's] of part L's. Three kills, 3 Part L's.