He said, 'By God I am not guilty'.

He swore by God that he is not guilty.

He swore by God that he will not guilty.

He swore by God that he was not guilty.

All are correct

All are wrong

Direct Speech: He said, 'By God I am not guilty'.

Indirect speech: He swore by God that he was not guilty.

Present Tense changes into Past Tense.

He is changes into He was.

Use second form of verb (swore)

Remove inverted commas.

AGM 30.4.23