I have posted instructions on how to cross-build and install your own kernel
not too long ago, see:
https://lists.debian.org/debian-sparc/2025/10/msg00103.html
I don't think it's particularly hard.
The upstream kernel sources also have a built-in way to build a Debian kernel
package, if you want to make your life a little bit easier:
https://kernel-team.pages.debian.net/kernel-handbook/ch-common-tasks.html
TL;DR:
export ARCH=sparc
make -j<jobs> bindeb-pkg
A little bit of preparation is still needed:
apt install debian-ports-archive-keyring
dpkg --add-architecture sparc64
cat >/etc/apt/sources.list.d/debian-ports.sources <<EOF
Types: deb
URIs: http://ftp.ports.debian.org/debian-ports/
Suites: sid
Components: main
Signed-By: /usr/share/keyrings/debian-ports-archive-keyring.gpg
EOF
apt update
apt install gcc-sparc64-linux-gnu libdw-dev:native libssl-dev:sparc64
