It would be useful to understand what kind of physics and interactions you need in your game. You could make a simple list because if you need full physics and collisions it is better to go 3D, maybe moving 2D flat elements in a 3D world to fake 2.5D.
Otherwise since you have to make custom physics, find out what you really need.
Probably you will just need small jumps (you can use Tween to simulate them) and play around with collisions (many times a collision shape tailored around character's feet to interact with obstacles can do a lot).