You can install any control panel on your existing droplet but this might cause issues for the website that are already configured on the current droplet.
I will also vote for a new droplet with a clean installation of any control panel of your choice. In this way you will prevent any issues for your live sites.
There a lot of web panels available (most of them are free to use) that you can choose in order to manage your server. What I can recommend is to try them as in this way you can see if it really suits your needs.
I will list the top control panels that I recommend:
CentOS Web Panel
and of course I need to mention Plesk although it’s not free it’s been available for a while and it’s always been the main cPanel competitor. It has a lot of build-in features and a lot of plugins that you can install and you can always reach to their support team if needed which is something that might come handy in time.
You’ve now got some great options if you want to use a control panel with your server but you don’t want to pay for a license. I’ve tried to lay out most of the available options, but which one is best will ultimately depend on your individual needs.