hw4 and HalfCheetah pybullet/roboschool

Hi all,
I'm trying to figure out how to port the cost function of HalfCheetah MuJoCo  to HalfCheetah pybullet for model based RL. The state vector is not really the same for the two environments. 26-dim for pybullet instead of 20-dim. Any idea on how to implement the cost function in order to produce the same behaviour with the two environments ?