The GPL (and other FOSS) sources from Archos have been checked into git for convenient patching and access:

There are efforts underway to get different kernel versions running on this hardware, e.g. nicktime has

Notes and status on Nicktime's kernel
You can try this kernel out with any image from at /images/archos_gen7/37/
Untar image as root to internal flash, e.g. if device gets mounted on /media/ARCHOS5:

cd /media/ARCHOS5
sudo tar -xJf path/to/image/image.tar.xz

update menu.lst with
flash new