No you need the Plugin version to match nopCommerce version. Upgrade the source code Check on the Marketplace for the latest version or contact the original plugin developer
Yes if you have the source code. Use one of the existing plugins as a template and change/add you functionality There will be quite a few changes including Database and Async processing
It will take you many weeks if not months to convert the theme and any plugins and test and debug it How much is your time worth ? I would be buying a new version of the Theme and plugins. Then its done instantly.