You could: Download the right package from http://maemo.org/packages/view/kernel-power/ (match the backup dates with those on the kernel) Extract the kernel. Restore rootfs & optfs then shutdown. Then use Flasher to flash the kernel only.
sudo flasher-3.5 -F zImage-2.6.28.10-power50.fiasco --flash-only=kernel -f -R
flasher v2.5.2 (Oct 21 2009) Image 'kernel', size 1741 kB Version 2.6.28.10-power50 Suitable USB device not found, waiting. USB device found found at bus 002, device address 008. Found device RX-51, hardware revision 2204 NOLO version 1.4.14 Version of 'sw-release': RX-51_2009SE_21.2011.38-1.002_PR_002 Sending kernel image (1741 kB)... 100% (1741 of 1741 kB, avg. 27216 kB/s) Flashing kernel... done.
sudo flasher-3.5 -F RX-51_2009SE_10.2010.13-2.VANILLA_PR_EMMC_MR0_ARM.bin -f -R sudo flasher-3.5 -F RX-51_2009SE_20.2010.36-2.002_PR_COMBINED_002_ARM.bin -f -R sudo flasher-3.5 -F zImage-2.6.28.10-power50.fiasco --flash-only=kernel -f -R