Need help reading a feed.

Using RSS Feed Post Generator, current versions Jan/Feb 2021. The feed I am reading doesn't validate, and I can see it is missing the basic XML Declarations for the data - the feed just gives the data, no declarations. Can you suggest a way that I can modify the data I am reading, server side, to add the declaration and then provide it via a new URL for RSS Feed Post Generator. I think I need to add some custom PHP (at my site) to read this data and then deliver it to Feed Post Generator properly delivered with XML Declaration and RSS feed information. Thank you