Do you have a coin to flip ?. It could be original and might not be.
If it's being represented as an original numbers matching car then the seller needs to provide proof of this.
If you have to do the research, then you need to be armed with knowing where part numbers are cast, where date codes should be stamped, how to decode date codes, what to look for as far as bolt head markings, asking the owner if you can scrape off dirt grease, or whatever to get at the marks, and so on.
There are some things looking at the one picture that has me leaning towards original BUT a LOT is still covered up and an unknown.
You never know too, with how that one picture is, it was stored years ago because it was stolen ?. Take that into account as a possibility.
You really need more pictures and information than just the one photo.
Jim