New guy with possibly-too-hopeful plans

[FeralSlug]

Hello, everyone. My name is Bryce and I'm currently a high school student near Atlanta, Georgia. My current car, which is technically my dad's as of now, is a dark red 2003 Ford Focus SE. However, this is not my project car. All I plan to do to it is get rid of the amber lights and tint the tail lights, for a grand total of about $20 out of pocket, since I'm almost immediately going to turn around and sell it. I'm going from 34mpg city to 13. That's simply because some British engineers at some point decided to make my favorite car into a tank, rather than use some lightweight materials and whatnot. But whatever. I want a 1988 Jaguar XJS convertible, and can get one easily for under $2000. Now, I know how these have a tendency to be absolutely horrendous and don't seem to work... ever. But that's beside the point. My immediate concern is not having the money to keep it running when it needs excessive amounts of fuel all the while. I know there's a lot that can be done, but I don't want to do anything at all that takes away from the car's natural beauty. So basically, I need to modify the engine and almost nothing more. My goal is 25mpg city, but I'd love to go higher, and as long as I break 20 I'll sure be happy. I guess before I get too excited, can you guys fill me in on whether or not this is a reasonable expectation? And whether it is or isn't, where do you suggest I start?