OSDev.org

Apologies for resurrecting a dead thread, I figure it's worth a reply in case this helps someone.
No one has addressed the fact that you shouldn't be compiling your kernel into the UEFI bootloader. This statement suggests that OP is a little confused regarding the bootloader's purpose and how it differs from the kernel itself. UEFI is not for writing a kernel, it's for writing a bootloader to load the kernel. The bootloader's job is to set up the initial environment for loading and executing the kernel. While the kernel is highly coupled to the bootloader, they are separate programs entirely. The UEFI bootloader should be compiled as an EFI executable, which then loads the separate kernel executable in whatever binary format it is in.
Regarding OP's original question of what you need to rewrite in your kernel: This is a really pedantic answer, but... nothing. Maybe. Most online tutorials suggest using GRUB and writing your kernel to adhere to the Multiboot standard. An advantage of doing this is that it would theoretically be compatible with any bootloader implementing this standard. If you roll your own bootloader you're going to need to decide for yourself what state your kernel expects the environment to be in.
If you're having trouble with writing your kernel, UEFI won't offer you much help. Writing your own bootloader in UEFI will not simplify any part of the kernel development process.