Its an interesting concept.
Winning needs to look different from losing. Perhaps flip the defeated critter upside down or make it a pile of parts?
Dragging all the pieces around gets tedious eventually. And its not very clear how good a part is. How about showing the level on the part? And about something like you pick out the parts you want to put onto your creature and then it eats the rest?
Now its win, mouse over everything, drag drag drag drag. Something like win 'swap level 3 with level 5 hands, eat the rest' would be a lot smoother. Some randomness in the enemy part levels then too so you can be excited about finding level 6 hands for example.