NB--The original 904 made rather less than 200 HP (the most powerful
race motor had 192 HP, according to
here), and quite a lot less torque I believe. Very peaky powerband, too, if my buddy's four-cammer Speedster is any example. It shouldn't be hard to find a diesel that would be roughly comparable to that. In fact, if you're willing to trade off the high-RPM power, I'd bet that any recent-ish TDI-type powerplant will do. And many or most would probably even have better low-end grunt for more flexibility and perceived acceleration.
The later 904-6 cars had an early form of the 2.0 liter 911 engine that made up to 210 HP, but the power was still somewhat peaky.
The car will be light, even with a strong and heavy diesel mill in it. So you won't need much power to give excellent performance.
The aero could definitely be better, but they sure are handsome cars... Just don't let yourself think about the possibility of an SUV hitting you. And don't think about driving it in actual hot weather; an acquaintance of mine co-drove one across the country once, and it was the next best thing to torture. They'd get bags of ice at a gas station, and just dump the ice on themselves. Not exactly fun; the car had no AC and no insulation, and very little in the way of openable windows, so the Texas sun turned it into an oven.
-soD