I believe you'll have to take this on the chin...........
The car was, and is, her property.
She instructed you to scrap the vehicle - you didn't.
You have fixed HER car at your own expense.
She is now aware that you failed to carry out her instructions and wishes the car to be returned. As it is her property you are obliged to do so.
However, there could be a case that YOU are still owner of any parts you have fitted, and if she is unwilling to pay for them you could remove them. As her car was unfit to run previously, you'll only have returned it to its former state...........
You'd be best advised to get Solicitors advice on the legal minutiae of your situation.