I needed a Manjaro ISO with an old kernel to be able to run the live environment and install Manjaro on an old computer not compatible with latest kernel.
It is possible to extensively customize the final ISO, I found the Manjaro WIKI page about this topic but I just needed to replace the kernel in the current Manjaro KDE ISO, so the solution is as simple as running the following command
buildiso -f -p kde -k linux419 -b stable-f means build the full ISO, not minimal
-p kde means to build from KDE desktop profile
-k linux419 means to use kernel 4.19
-b stable means to use the Stable branch
Here are other options from buildiso
buildiso -h
Usage: buildiso [options]
-a <arch> Arch [default: x86_64]
-b <branch> Branch [default: stable]
-c Disable clean work dir
-d <comp> Compression used for build ISO: gzip, lzma, lz4, lzo, xz, zstd
[default: zstd]
-f Build full ISO (extra=true)
-g <key> The gpg key for sfs signing
[default: ]
-i Initialize iso-profiles repo [default: master]
-k <name> Kernel to use
[default: linux54]
-l Create permalink
-m Set SquashFS image mode to persistence
-p <profile> Buildset or profile [default: default]
-o Enable office installer module
-q Query settings and pretend build
-r <dir> Chroots directory
[default: /var/lib/manjaro-tools/buildiso]
-t <dir> Target directory
[default: /var/cache/manjaro-tools/iso]
-v Verbose output to log file, show profile detail (-q)
-x Build images only
-z Generate iso only
Requires pre built images (-x)
-h This help
It is a first step into the world of creating a custom ISO, here with only a slight modification, but it is possible to add or remove packages, set some default settings and much more, but that's for another day.