Another possibility is a pair of games based on the Song of Blades and Heroes engine by Ganesha Games: Samurai Battle Robots Royals (focused on stumpy robots), and Mighty Monsters (for kaiju movie monsters). Both let you design the robot/monster by choosing sections (head, body, arms, mobility) and assigning special abilities to each part. During play, you decide what part to activate (ex. Legs for making a move), then roll against a target number to carry it out. Fail too many rolls and the turn goes to your opponent...
The rules will let you build what you want, but whether or not you would enjoy the game mechanics is another matter - Ganesha Games titles tend to be a love it or hate it thing.