Create Chrome extension base on already created one

There is Chrome extension which parses data from popular car dealer sites and automatically fills data into specified sites submit form fields. This extensions is built via JS and JSON files. Parsers should not be changed only target site should be changed so this extensions should fill data in my sites submit form. I just need to make few edits to make it work with my site. I know that this is not hard task and will not take much time. I know basic JS and may find solution by myself, but I jsut have not time to dig into it.