Can I replace just the starter cover or do I have to replace the whole bell housing?

 

Those starter drive covers were rolled into a groove in the clutch cover. AFAIK no one sells replacements, and a used one would likely be destroyed removing it. The cover is totally a dust cover, not structural, I'd just wrap it with silicone tape (self-fusing tape) or a piece of sheet metal with a hose clamp around it.