Are you doing this inside of a browser? a good idea would just be listening for any links to the emote site:
http://static-cdn.jtvnw.net/emoticons/v1/
And replacing those images with a link to a custom ( or even local ) image.
If you’re using some kind of irc and want to parse chat;
I think the best bet is going to twitchemotes.com
and downloading the emote files from their API, to get all emote “text” - and replacing those with an image.
EDIT: My bad!
API of twitch for all emotes