Spoiler nut missing - can I get a replacement?
 

I found this at the junk yard and 3 of the nuts rusted out when I removed it.

Can I simply put 3 in or do they need to be inserted from the inside?

What about magnets or double sided tape?

https://cimg3.ibsrv.net/gimg/www.che...7c32f6f657.jpg

They look like rivnuts. They work sort of like pop rivets.
https://www.boellhoff.com/ca-en/prod...uts-rivnut.php

It looks like it's in horrible shape. Unless you are a good body repair and paint pro, I wouldn't waste the time. Look for one in better shape.
Those cracks appear to be all the way through. Not an easy fix.

https://cimg6.ibsrv.net/gimg/www.che...d39bee82e5.jpg

Well I got it and I'm going to paint it as best I can and use it! Some bondo or glazing putty should fix the cracks?

Lots of hard work.
The crack is especially annoying. The spoiler is designed for a significant wind load, its fastening must be so strong that it is recommended not to fasten it to weak surfaces in order to avoid deformation of the metal.
If you do not eliminate crack, your spoiler will not hold tightly to the body. After a short time, the air flow will tear off the spoiler. Your result: lost spoiler + body damage. If I judged your picture correctly, this spoiler was thrown into the landfill because it had already been torn off the car once. The body of the spoiler could not withstand the air load and cracked.
What to do: To be honest, I would return this thing back to the landfill. I can't imagine how it can be brewed or glued.
Of course, a way can be found, but you will spend a lot of time and labor (and possibly money), and the result will not be guaranteed.

That hoop spoiler used two fasteners on either side. You could you possibly install a new riv-nut in a new location? Double sided tape as a backup is an option.

Band-aids don't repair cracks they cover them. Maybe a plastic welder would work.

The spoiler is ABS plastic, so use a product for that material, the double sided tape would only be for mounting it to your lift gate. Not to repair those cracks

https://cimg9.ibsrv.net/gimg/www.che...18f2673ffc.jpg

I see three cracks.
Two are very developed and form a torn piece of spoiler.
The third crack is in its infancy.
Events developed as follows: the spoiler material had a defect or its fastening was not tight. As a result, the powerful airflow caused the vibration. Vibration caused a break in the body of the spoiler - cracks. As a result of the cracks, the riv-nut loosened and broke off from the spoiler body.
How to treat theoretically: eliminate cracks to the state of the material that meets the manufacturer's standards. Return the hole diameter of the lock nut to its original state. Replace riv-nut.
In practice, only the third point is really possible, but it will not bring the spoiler into a working state.
Given that the spoiler for HHR is anoptional option, the question arises: "How urgent is its installation? Is the effect of installing a spoiler worth the effort?"
But, of course, if the desire to try is strong, you need to try. Don't forget to tell us later how your heart calmed down.

I doubt those cracks developed while it was on a car. At the speeds we drive and the spoiler designs used, there is little downforce.

More likely it was damaged during removal from the car. What probably happened was the other bolts were removed and this one was stuck in the nut enough so that the nut spun in the plastic. The person removing it decided to just pull on the spoiler until the nut pulled out completely, putting stress on the plastic around it and cracking it.

Plastic welding would work the best. If it's ABS (as Oldblue says) it can be plastic welded.

From a plastic welding source (pay attention to the last part about surfaces that don't mate tightly):Many plastics, including ABS can be chemically welded. The result will be as strong as the original plastic. Thin parts may not have much surface area to join, and you may see a visible "melt" line at the repair.

Joining broken parts is essentially the same process as building things with a material like Lucite. You put the pieces together with a tight-fitting joint, then use a micro-pipette or syringe to apply solvent to the joint and nowhere else. Capillary action draws the solvent into the joint (which is why it needs to be tight-fitting, and why not much solvent is required). It isn't necessary to dissolve a huge amount of the material into a blob of plastic that runs together. The welding happens right at the mating surfaces.

For repairing broken parts that fit together tightly, a common technique is to fit the parts together, then use tape on one side to hold them in place. Apply the solvent on the other side. Once the joint is dry, remove the tape and check that side. On a really thick piece, the solvent may not wick all the way through the crack, in which case you can add a little solvent on that side.

If you're repairing broken parts that no longer mate tightly, you need a different method. You will need a filler material for the gaps. That's typically done by dissolving some of the same type of plastic in the solvent (most plastic cements are a mix of solvents and some dissolved resin; for welding, the cement needs to contain appropriate solvents and the same type of resin). Apply a thin layer to both broken edges, then push the pieces together.