In this case it's not really just a material thing. Cars are material expressions of an ideal and some are like a piece of art. There are cars I have a strong response to and they are also cars I loved and got to ride in when I was a kid. For me the Porsche 911 and the early 60's Corvettes are like something from a dream.
Your fiance should never be allowed to modernize the interior. That's insane. Obviously. And now he thinks you need to sell it? Breaking up over a car sounds really bad, but you guys need at least to see a counselor. Fiance sounds like he has a need to control. He's trying to claim ownership. He's making plans to make it different. Maybe he should just pee on the wheel and mark it like a dog?
Lou sounds like a great guy and I'm sure it gives him great joy to share this car with you. The excitement you have shown for it must have been really special for him to see. Whatever happens you can't ever let fiance change the car. Frankly, I'd get in that car and burn rubber on down the road. Leave fiance in the rearview mirror. Maybe I'm being too harsh, but I just really see this as a terrible personality trait. The right attitude would be happiness for you that you have your dream car. Not being able to properly drive it might sting a little, but all the more reason it's your car. You've been given this "dream" and he wants to ruin it and take it away from you.
And I might sound crazy in this post but I understand and you don't have to apologize at all for how you feel about owning the car of your dreams.
Best of luck whatever happens with this.