The two approaches I use in situations like this are:
1. Check the API Explorer, specifying Unsupported version, and review the actions tab for the object type of interest. Sometimes, you will find a relevant endpoint that can be used in a Custom API Call module.
2. If Step 1 fails, perform the action in browser and use the browser dev tools/network pane to inspect which endpoint Workfront uses when performing the action in the UI. (These endpoints sometimes only work in a HTTP module.)
I see no relevant endpoint published in the Unsupported API version, and "Copy" isn't an action that can be done in the UI, so I believe your described approach is the only option for your scenario.
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