Reactome does not have a mapping file to KEGG. The best reference for such a comparison is ComPath (reference here), however this resource may not be up to date.

As an alternate approach, users can try the method used to establish links from models in the BioModels database to Reactome. For each model, we took its gene set, ran a gene set enrichment analysis against Reactome using the Reactome API documented here, and then added links to the Reactome pathways below a certain p value cutoff. 

You could use that method to identify Reactome pathways that are related to a given KEGG pathway, and consider pathways with no match below your p value cutoff as not matched. The Reactome analysis interface is fast, each query should return in a few seconds, so this is perfectly feasible for all of KEGG. We'd be happy to communicate further on this.