I had this problem with 1 screw and ended up using a small rounded file to wear away at the head of the screw until there was so little left that I could pop it open. Problem with this is the remainder of the screw is stuck in the other half. Was manageable for me as the case could still be closed with 1 screw missing.
You'll have to get someone with a extremely fine drill to drill into the screws in an anticlockwise direction so that it has the potential to unscrew it while drilling.
Honestly if you have a few threaded you'll need a replacement casing as well once you get it open.
If it was me i'd sell it for spares parts on eBay as it's not really worth the repair investment.
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Stripped threads or head?
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