You can get a whole lotta inspiration here https://www.hometalk.com/search/posts?filter=Bird%20Baths
Here's a good idea: https://www.addicted2decorating.com/concrete-birdbath-updated-with-mosaic-tile.html
I didn't want to keep mine as a bird bath with water, so I turned it into a fairy garden. I change it out seasonally with fun pieces.
A picture would help. We do not know what it is made of or if it painted. Have you tried just scrubbing it out. A power washer is helpful. You can scrub out with a product containing bleach. But be sure to rinse very very well.