Yes you can.  If you are sufficiently skilled you can use the tftp method described elsewhere to install any .bin firmware image at any time.

If you are not comfortable with the tftp method you can install the original firmware from the node Administration page.  Connect your node's LAN port to your laptop and its WAN port to your home network.  On your laptop send your browser to http://localnode/  Click the Setup button, enter your username (root) and your password, then click the Administration link at the top.  Click the Refresh button next to the Download Firmware list.  Then in that list select Linksys-WRT54G or Linksys-WRT54GL depending on the model of router you have, then click Download.  The Linksys firmware will be downloaded and installed.