This site has info that explains how to build a RISC-V Linux system from scratch. It also has a section that explains how to install a linux kernel update for this build. This system may be structured differently than the Ubuntu release, so the info may not be exactly correct for your system, but it might be helpful. https://github.com/carlosedp/riscv-bringup/tree/master/unmatched
I think the boot image file alone is not enough, you also need the dtb file and the module files installed. Depends on how the image is constructed, and whether you enabled modules, and maybe other things.