only contains "mirabox" and "rootfs.tgz"
So it contains only the rescue kernel and rescue root file system.
"mirabox" is a kernel image; is that the one to use
Although it is obviously compatible with the Mirabox, I cannot say if there is some functionality missing or not.
create my own ubi image from the untar'ed rootfs.tgz
That is possible (with the use of 'ubinize' after creating your own rootfs image file).
Rather than create your own based on the 'rescue' disk it would be safer to flash the stock OEM software that is installed on the Mirabox when you received it (which may in fact prove to be the same).
Which brings us full circle back to your original question...
What kernel and rootfs image, where do I get those?
1) If your Mirabox is working, you could use that to make backup images...
2) Google for them...
3) Or re-read ckilmer75's first post... (http://code.google.com/p/mirabox/downloads/list