I would try checking the vehicle impound. That is where I found a car I modded and then left it during a mission on the side of the road.
I checked there and, annoyingly, nothing. There's nothing else even in the impound, so it couldn't have been overwritten.
I have no idea where else to look. I've checked his house's garage, the big storage garage and the impound. The only thing I can think of trying is looking where I last had it before those storyline events kicked off, which is the movie studio.
If it's not there then I really think it's gone, which is at least 150k down the drain.