In the case of defective verbs, how are they changed from the direct into the indirect form of speech?
In case of a defective verb you use its synonym which has all forms.
Direct speech: He said "I must go to work on Saturday."
Reported speech: He said, that he had to go to work on Saturday."
Direct speech: She said "I could not do anything with it."
Reported speech: She said, that she had not been able to do anything with it.
In the examples I used have to as a synonym of must, and be able to as a synonym of can.