The car looks pretty nice. I assume the O.P. thought so or he wouldn't have bought it. But it sounds as if the buyer's 2 main concerns are whether or not the car is stolen and the fact it's not numbers matching.
We each have criteria we use when we purchase a car. In today's world a numbers matching car tends to have a higher value than one that's not. I don't know if the sellers intentionally misled the buyer. They may have simply repeated what they were told when they bought it and never checked it out. Regardless, it does have an impact on the car's value. If it's truly important to the O.P., he's unfortunately shot himself in the foot by not checking it out first. He's not any different than most of us others.
Obviously none of us wants to go through the stress of losing our investment if something shady has occurred. We'd each go through that 'Damned if you do, damned if you don't' feeling when trying to decide what to do. On the one hand he doesn't want to be a part of having screwed over a theft victim (if that turns out to be the case). - On the other hand he doesn't want to lose his cash when he was innocent of any wrongdoing.
If I were to offer advice it would be to have him try to do his own investigating to find out what he can before contacting the authorities. If he finds evidence that the car was stolen he will know that he should take steps to protect himself and his investment (perhaps BEFORE contacting the law). That may mean pursuing legal action against the seller or the shop where the car originated from. My main fear would be having the car confiscated and out of my control before I was able to take steps to protect myself.
I do suspect there's a high probability that this case will boil down to being nothing more than a car pieced together from several others and that nothing illegitimate has occurred. If that turns out to be true then the biggest concern will still be whether or not the O.P. is OK with owning a non-numbers matching car. From the picture, it looks like it's worth at least what he paid.