Is there a such thing as Magnum V8 Cummins Diesel or is only Hemi?

There is no Magnum V8 Cummins diesel engine. Magnum is a brand name associated with Chrysler/Dodge vehicles, and their V8 engines are typically gasoline-powered (Hemi being a prominent example). Cummins is a separate company that manufactures diesel engines, primarily for heavy-duty trucks and some cars, but not in a Magnum engine configuration.