What are the necessary DICOM header fields for import into 3D Slicer?

Slicer should be pretty tolerant of missing dicom ags, but as a baseline you could consider the ones listed here:

https://www.slicer.org/wiki/Documentation/4.10/Modules/CreateDICOMSeries

Or, if you load the numpy data you can create a volume node and then export it as DICOM via the Data module.