Friend, I am suggesting a copy of Persian Chachnamah and its English translated and published in book form is available in Aligarh University Library. It is a translation of Arabic MSS named Fatahnama-i-Sind and is one source of information on the events connected with Kasim-Dahir battle fought in 712 AD.
I have shared what I know on the issue, hence I will not be in a position to attend any further questions on the nature, origin, availability non availability, authenticity, non authenticity and so on regarding Fatehnama/Chachanama.
Yes, you are welcome to contribute and share your views on the battle of Brahmanabad between Kasim and Dahir in 712 AD!
Thanks and regards