Some of the user are unable to re-write IMEI number after flashing to stock ROM. Although SN Write tool is provided with stock ROM but I don't why they are not using it :D Anyway with this little steps you can easily re-write your phone IMEI number without PC requirement.
Follow this step to get your IMEI back on any MTK 65XX device, requires root.If this does not work then I highly recommend to use SN Write Tool.