Actually you can use sysupgrade on Stock Firmware.
Follow the steps listed on https://openwrt.org/toh/linksys/linksys_wrt32x under Return to Stock Firmware/CLI.
Return to Stock Firmware
Return to OEM
CLI
Login to OpenWrt via SSH (DropBear / OpenSSH)
Linux/BSD: SSH client built in
Windows: Download PuTTY
Download/Upload firmware image to router
Download via SSH Terminal:
cd /tmp && opkg update && opkg install wget && wget <Image-Link>
– OR –
Upload via Client Machine Terminal:
scp -r -i "C:\OpenWrt.ppk" -2 -scp -P 22 "D:\Path\To\Firmware.img" root@192.168.1.1:/tmp
For PuTTY, change: scp to pscp -scp
-scp = force use of SCP protocol
-r = copy directories recursively
-i = private key file for user authentication
-2 = force use of particular SSH protocol version [2]
-p = connect to specified port
Flash image
SSH Terminal:
cd /tmp && sysupgrade -F -n -v <image-name>
-F = force write, required to bypass metadata check
-n = do not save configuration over reflash
-v = more verbose