I followed this https://github.com/openwrt/openwrt to get my environment ready. How can I get the source code files specific for my WRT1900ACS running OpenWRT 19.07.3? Then after doing so, shall I follow the above Wiki link for quilt?
Looks like it, yes. But I do not use the router so I cannot guarantee anything. Make sure the name matches the name of the 19.07.3 file you would have downloaded.
Only you know what packages you installed. But as long as you can use ssh you can install LuCI and all the other packages you need. opkg list-installed will give you a list of installed packages and then you can compare the list before and after.
I see. Well mo problem about any extra packages installed by me. I am talking about the basic packages that come with a new fresh images of installed openwrt by default like hostpad, pppoe service, dnsmasq, essential packages, etc...