Sure. Feel free to author the upgrade:
Just figure out the needed change to Makefile and related files, author the changes, compile and test them, and then issue a Pull Request to get your changes accepted to the source repo so that buildbot can compile the updated package for opkg.
OpenWrt is a community effort. Packages need people interested on them to do the upgrades. Apparently there has lately been nobody interested in keeping webui-aria2 updated. @kuoruan was interested in it some years ago, but has not been active with that package lately.