The art and concept are really fun. This reminded me of a 2000s Newgrounds game.
The controls could use some tweaking to in my opinion. It would be nice to be able to use the shop with mouse instead of having to tab through options with the keyboard. Having different parts mapped to random keys could be interesting (especially with more parts), but I think this game's biggest strength and potential is in the part selection itself. Physics based pieces were a smart choice, there's a huge possibility space there. I could imagine a version of this where the player doesn't even have input during the fights (sort of like Horsey Game, which I'd recommend looking into since it has some similar mechanics).