What’re the UI customisation that could be done on custom script report?
From web search, I understand that we can do Filter & Message (A small one under title).
Are there anything else?
Can we change the title to fields from the Report doctype?
Can we add description somewhere else?
You can pretty much change the entire UI to anything you want if you just use a little jQuery to modify the css or append any html dom elements anywhere you need.
In the console if you look at cur_frm.fields_dict, you’ll find all the form fields available, and you can modify their df_properties like making a field hidden/read only on certain events etc.
If you’re looking for entirely custom pages built with a completely new UI, look into the Pages doctype.