I really must apologise, as I did not offer my sympathies for your loss in my previous post. That was really insensitive of me

. She was clearly a well loved chicken and you must be really feeling her loss even if she was not your bird.
I know it can be emotionally taxing thinking about cutting her open, but I find that it also gives me some closure and often I learn something from the process which helps me in the future or helps me to help someone else via this forum, so it can be a beneficial process if you can overcome the mental obstacle of doing a necropsy yourself.
Obviously, since this was not actually your hen, you would need to seek permission from your neighbour before going ahead with that. If you decide to do it and get the green light from next door, there are a few of us that have quite a bit of experience at this and would be happy to guide you through it.