That's too bad. I saw a forum where a guy said he installed JLime on his 620 in 2016. He gives a download link to the defunct JLime website.
It is a long shot, but you might try contacting him to see if he still has the installation files available. You may need to register on that forum first.
http://www.vcfed.org/forum/showthread.php?51953-Jlimed-my-HP-620LX