This will be easy and also a giant pain to do in equal measure.
I have already ported the custom spawn code and stuff for the dimension - yay!
The actual mobs are easy themselves with a bit of render tweaking etc.
The custom AIs have quite a few things with regards to Chunk co-ordinates and blocks that will need changing to the new system.
A few mobs have custom projectiles etc which also need porting.

iirc there are around 60 (maybe more) mobs and entities that will need to be ported.

This will be a slow(ish) process...
I think the 1st mobs should be beetle lava, beetles, rhino beetles, titan beetles, stag beetles and possibly something fun (like wasps). The beetles listed above are all connected to the larva and will need doing at the same time really. Each beetle (apart from plain ones) have utilities attached to their AI's and some custom code that will be tangled up in many classes no doubt :(

Lots of the other mobs will be simpler to port across - so if I get annoyed with the grind I'll work on the easy ones 1st ;)