PE headers have a field for machine type, you're looking for Machine in FileHeader: https://learn.microsoft.com/en-us/windows/win32/debug/pe-for... and https://learn.microsoft.com/en-us/windows/win32/debug/pe-for...

EDIT: Or maybe what you're looking for is hybrid PEs? https://ffri.github.io/ProjectChameleon/new_reloc_chpev2/