this fella was able to disassemble the 520 -
https://ifixit-guide-pdfs.s3.amazonaws.com/pdf/ifixit/guide_92578_en.pdf
Here is the part
https://www.ebay.com/itm/Garmin-Rep...304961?hash=item3b28d7d941:g:ur8AAOSwNe9cRpMo
or
https://www.ebay.com/itm/New-Shock-...art-replacement-repair-lens-DIY-/253459731155
looks like the 520 sells for around $150 on fleabay.
GLWS/R