I'm glad Pieuw agreed. Has he said anything about what to do with levels that don't work in part or whole?
I'm afraid many levels will give you trouble, as I had fun exploiting a lot of Lemmini's weird mechanics... you can do what you think is best, I give you carte blanche. Open source PimoLems!
Levels like "Fish Fry" will certainly need some reworking and thinking about a clever different solution, but as far as the standard level goes I just remove time limits where it makes no real sense (it is just there, because Lemmini must have one), looking if 1pixel gaps that have been created in the conversion, adating a more fitting level size (+ avoiding building to the side solutions) and direct drop prevention, create a replay and see if everything works or little changes are needed.
No worries! I will find a clever solution for the "problematic" levels