Quite simply put there is no easy way to make a plugin that’s made for WordPress to work without WordPress.
The amount of effort to convert a plugin is dependant on the complexity of the plugin and on how much it integrates with WordPress.
Most plugins will make use of native WP functions, DB tables, hooks and filters. There is a lot there that you would have to consider.
Either way you look at it any plugin to make it work standalone would require a lot of time coding, to be honest I just wouldn’t bother.
You would be better finding scripts that work standalone and do the functions you are after or build one from scratch for your needs.