How get the _source data when the _source is disabled?

I have one index whose _source is disabled. Now I want to migrate data from this index to a different elasticsearch cluster ? The issue is I only get metadata fields while searching. How can I retrieve and transfer _source also to the new cluster ?

You will need to reindex the data from it's original source, ie from outside of Elasticsearch. There's nothing that Elasticsearch can do here to assist you sorry to say.

I think you could use snapshot and restore to move the index to another cluster but this will only move the indexed fields and will not restore the _source which is disable from original cluster / index.