I put together some links which should contain useful answers on how to customise the excepts:
In short you apply a convention to tell the indexer what it should use to work out the excerpts. Don't forget to reindex when you test :)
David has given some good suggestions for you to review. I think to have complete control you may want to extend your Page Type to inherit ISearchContent, so you can control exactly what gets indexed.
You can exclude fields using the client conventions as documented here : https://world.episerver.com/documentation/developer-guides/find/NET-Client-API/Customizing-serialization/Excluding-fields/
I'm looking to remove a property included in the Excerpt on the Episerver Find search results page.
Any help appreciated.