The best thing to do is to contact the original developer and see if they are able to release an updated version of the module that is compatible with Mendix 9.
As you said the other option is for you to convert it to a newer version. Usually, this is a case of opening it in the last long term support version of Mendix the module was written in and saving it. Then it’s opening that in the newer version of Mendix you wish to use. Depending on how the module was built, you may find there are compatibility issues with the APIs used. One big one is the deprecation of Core.execute in Mendix 8.6.
I hope this helps. Good luck!