The way I've always looked at it, it's far easier to do anything if its not a wordpress plugin. If you have the plugin, why do you need it remade anyway? (Unless you're getting rid of Wordpress, in which case I understand completely)

It's not my plugin. I want to take the same idea and use it on my website so other people can use it. The other plugin makes it so to use it, you have to install the plugin yourself.

If you can get it from wordpress.org the code is open source
You can copy it, rebrand it and redistribute it.
You just cannot sell it.

Should take a few minutes, depending on the complexity of the plugin.

If you have to buy the plugin, the complexity is the same.
But you would be stealing it.