First of all, make sure that the lack of a braced front leg is the real problem with your action. There are plenty of bowlers who manage to bowl quick with a bent front leg, like Dale Steyn:
If you have decided this is what you want to do then I think you're going to have start from scratch and build your action up again from walking pace. Walk slowly through your action, get used to the feeling of bracing your front leg and coming up and over it. You don't even need to have a ball in your hand, you can do it in your living room or wherever, it's just about getting your body and mind accustomed to a braced front leg.
Then slowly work your way up to using a ball, coming in off a brisk walk, then a slow jog and gradually build up from there until you're bowling at 100% again. Remodelling actions is always a tricky process, something which I am about to learn if I'm really going to try and bowl left handed