3D Troubleshooting avatar build

This is my first time in the Unity forums so not sure if I'm in the right spot, please let me know if I need to relocate.

I've been building a custom avatar for VRChat for a while now, and as I finally get to uploading it in unity, I've been hitting a lot of problems. I've solved most of them, but a couple of them are still persisting. Primarily it seems some of my meshes have vanished when imported to Unity. The model was built in Blender from a free template.

What it's supposed to look like:
View attachment 1187530

What I wound up with:
View attachment 1187533

Whatever happened, I lost the mesh on the helmet triangle and ridge, and the eyes are either lost, or somehow out of place (I tried re-running this several times, and on some occasions I could see the eyes blip in while moving the model for a moment, but nothing lasting).

Any advice or tips for troubleshooting would be most appreciated. Let me know anything else I should be posting up here to help with the process.