Looking at this specific case, quite a bit more work than usual is needed as the used SoC SiFlower SF19A2890 isn't supported by OpenWrt at all yet. While drivers for basic features are probably available in source in the Linux 4.14 source tree, all more advanced features are binary-only and will work only with that specific outdated Linux 4.14 kernel. See all the binaries in the sf_* folders in:
Now given that you find the complete kernel sources (ie. including the sourcecode for drivers of ethernet, switch, wifi, etc.), that probably still wouldn't help too much as it's Linux 4.14. And because these drivers are most likely not built using the kernel subsystems (ie. mac80211 for the softmac-wifi cores, dsa for the switch, ...) but rather giant monolithic beasts impossible to ever run on any other kernel than that vendor one and also not trivial to "port" (rather: rewrite based on knowledge gained from...) to recent kernels.