After he had taken great pains to finish it, I took upon me to revise and correct it, that nothing might pass but matter of fact, apparent against them by record. It is very little he hath inserted, and that necessary, to show what their offences were, what people, and of what condition they were. The whole proceedings and evidence against them, I find upon examination carefully set forth, and truly reported, and judge the work fit and worthy to be published.
I'm having a difficulty of understanding the bolded part. Is it okay to understand there was an omission of 'is' between 'that' and 'necessary'?