Face ID parts are complicated and difficult to repair. To fully understand Face ID, you can check below.
[guide|128388]
And for a detailed repair process, you need to pinpoint the faulty component first. Whether the flex cable has been ripped-off or relevant component has missing. Hope the following repair guide can help you somehow.
'''Face ID Not Available because of a missing component''' '''on the motherboard:'''
[guide|136762]