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Never done it, but in the Videos I watched suggested that you can just stick the lens back on using the old glue. So heat up, open, clean or whatever, heat up, stick back on.
I would definitely do this, but my car was free and cost me 150€ so far.
So in your case, probably something from the [sika melt] line.(https://automotive.sika.com/en/solutions_products/exterior-adhesives/sikamelt.html)
No, Porsche offer the opportunity to purchase an entire headlamp unit.
Hot-melt adhesive as in a glue-gun?