Nooononono.. ALL YOU GUYS NEED TO SEARCH FOR 4.3 firmware's modem file and install it when you installed 4.1.2 rom.. so first install 4.1.2 onto your device and then look for 4.3 modem file and install that, too. This is what you need to do!! Your IMEI will appear then right. This is because 4.3 firmware converts your EFS version to EFS v2 (originally you have EFS v1) and 4.1.2 rom's modem isn't compatible with EFSv2.. SO ALL YOU NEED TO DO to look for 4.3 modem and install it with 4.1.2 rom!! This is 4.3 modem's code "XXELJ2".
I hope i could help you!
(firstname.lastname@example.org <-- if u have questions)