Glad you liked it! One of the most damningly accurate comments from the original compo stream was "we have Doom at home."
The enemies are in the source code, as the arrays submittable_enemy, rat_enemy, and medium_enemy. These are each 256 bytes, ostensibly 16x16, with each byte pointing to a tile in the background bank. They are rendered with fairly simple scaling, so any patterned tiles repeat, e.g. the treads on the rabbit tank things. The animated distortion is handled in enemy_rendering() below a comment reading 'apply wiggle.'
If you just want to hex-edit a ROM hack, they're visible around 0xF790 in the "finalish" release. Right above "WE MAY BEGIN AGAIN".
But the whole project is permissively licensed. You can do anything.