this is my how-to form a while back.
Cloning a N900 from backupmanager restore files
https://talk.maemo.org/showthread.php?t=100595
If you can find the posts by JeorgRW on how to directly extract the required kernel and directly cold USB flash a complete built and ready to use backup image including kernel you could even bypass the bootmenu stuff. He is still almost always on the Maemo IRC chanel https://libera.chat/ #maemo I think he is just Joerg there