You are going to need to get it apart and see what will fit. That is going to make a lot of your decisions for you. While I havent done an old Clearo Tank Mod, I have converted a Steam Crave Titan to a proper PWM and it required a lot of on the fly engineering and far more time than I would like to have spent on it.
The display will be the most difficult piece to address, chip and knob the second. I'm following along as I am curious about what you do. Please post pics.