tentative #1, to rebuild "virgin" 3490 (an upgraded 3760), but safely (keeping all the files, to recover at anytime):

PRE: MTP mode always ON

1- attach the USB and move all the files into a newly created directory on the nav (i.e. old-3490)
2- disconnect from USB and restart the navigator; it starts in english, but using the maps on the sd card, and also having saved places still present. Magically a Garmin folder is created and an XML files is present in it.
3- connect again USB. launch webupdater and let it go. deselect unneded ASR/text/languages.
4- copied gmapbmap from the old location to the newly created .system dir
5- disconnect from the usb and let it reboot, select your preferred language, etc.
6- now in my case, I found first issue: no text and no keyboard for italian, but all the other customization were present.
7- reconnect USB, to copy keyboard and text, from the old files
8- disconnect and let it reboot.

now i have to copy some of the files copied at the end of the upgrade 3760->3490 procedure, will do it tomorrow.
the unit is working properly, with some of the feature of 3490 missing, due to missing files.