Unfortunately, it is not there (yet)... There are email addresses on the IndLinux page for BolNagri https://indlinux.org/wiki/index.php/BolNagri, perhaps an author there might assist with porting it to an emacs input method.
However, you can always use the system input method (in my case on Linux xkb) with emacs.
In Linux/KDE it is possible to map switching input methods to
Ctrl+Super+k, as this does not interfere with an emacs binding. How this is done, is described here. This is, indeed, not ideal, as we would strife for a environment independent solution within emacs. So someone needs to sit down and port BolNagri to emacs...
A downside from the system input is, that the emacs key combinations don't work. For example, if you press
Ctrl+C emacs will register the
Ctrl+छ, and the commands won't work as usual. Thus, having a real BolNagri input method is desired.