good luck with reverse engineering. even with present day technology, it's hard to reverse engineer with similar levels of tech. for example. it takes years for china to reverse engineer russian equipment. and there's maybe only a 10-20 year gap in the technology or even less.
comparing us to aliens is more like comparing us now to romans. the romans just simply can't reverse engineer a F-22 or a nuke. maybe they can reproduce a Ak-47, and the ammo that goes with it, but anything that require technology that they never even dreamed about such as jets, rockets, titanium/aluminum they can't produce. and where are we going to get the industrial background and economics to build them??? that's assuming we survive the first wave. also their technology might be significantly different from ours, everything we have is based on electricity, they might have discovered something that doesn't exist in our part of the universe and base all their technology on that. also what about raw materials?? if the alien's home world is abundent in gold, and have everything made in gold and only works with gold, are we going to use the entire world's gold reserve to build a spaceship??
now if there's an alien race that comes to earth, they could totally bombard earth until it's a wasteland from outer space and we can't do anything about it. max we can send a space shuttle to sign the official surrender papers. it's not going to be like independence day where they actually come to earth and let you have a swing at them.