I would check out Hag's House
http://www.hagshouse.com/forums/index.php? and find some homebrewer's. We had a few that posted here, but I have not seen much activity on that for a while. They may be able to give you sources for common parts like a lens cover or Fresnel lens.
If you can't fix it, you may be able to repurpose it. These low end Chinese cams don't last long anyway. Back we had significant trespass issues, I would use dead cameras a decoys. I place them out in obvious locations and hide a working camera nearby. Trespassers thinking they got caught by the decoy will usually go to it to pull the SD card or destroy the camera putting them in a great position for close up pictures by the hidden camera.
Depending on the characteristic of the failure, the camera may still work with a shortened PIR range. If you use it in the field with an opening, it won't be long before it is dead because of water. I had something messing with my seedlings I was growing in containers on my deck. In a much more weather controlled situation like that, you can protect the camera from the brunt of weather elements and get some use out of it.
Another interesting thing you can do is simply seal up the PIR cover. You can use epoxy or whatever to keep the elements out. Don't worry about the effects other than sealing since you won't use the PIR for this. Then program the camera to take one picture a day at a certain time and put it over a newly planted field. You can then stitch the pictures together in a video editing program and create a time lapse video of your plot growing.
Some cameras have a feature that will let you set a delay period and take a picture when it expires regardless of the PIR. If you have a large enough SD card, you can turn this into a "PlotWatcher" style camera. You mount it high so it overlooks the entire field. You might set the delay at 5 or 10 minutes. When you retrieve the SD card, again you use video editing software to stich them together into a video. For techie folks, there are tools that can let you automate this. The result is a PlotWatcher like video that lets you see how deer enter, move around in, and exit the field. This can be important for choosing treestand locations. There are even some tools that will do frame by frame change detection that can scan through your video and create bookmarks when there is activity to make reviewing faster. Granted, this is a bunch of time and work to do it manually, but it is another way you can repurpose your camera to do something interesting.
Thanks,
Jack
