Welcome to the forums,
I'm very sorry for all that you have been going through with Chicharro.
Due to his fragile state, it's best to keep him on the small cage and to follow the recommendations from the avian vet to keep him warm.
In addition you can cover the cage's bottom with a small towel or cloth in order to cushion a potential fall, since you mention he has balancing problems. Putting a couple of perches at a lower position (near the bottom of cage) will also be helpful.
During this time you can supplement his diet with egg food as this is highly nutritious and is helpful to nurse sick and convalescent birds. In order to alleviate the respiratory distress and help with the breathing, you can create a little steam area for him. This can be done by placing the cage in the bathroom, closing the door and running the shower hot.
Did the vet give any vitamins or medication for the stroke?
With time, it's possible for Chicarro's leg to regain some if not all of the mobility.
There is medication that helps in achieving this and you can also help him out by encouraging to use his bad leg and to do some simple physical therapy exercises.
Knowing Chicharro the way you do, you are in the best position to make a decision, it will also be extremely important the talk you will be having with the avian vet regarding Chicharro's overall health condition.
If he has good chances for a recovery, then you will continue to help him on the journey to full health.
Even if by chance he doesn't regain mobility on his foot, he can still lead a completely normal and fulfilling life.
I'm sending my prayers and best wishes for your Chicharro's recovery.