What size frame is your 520? In the smaller frame sizes around that era, Trek used a 1" fork, while using a 1 1/8" in the larger frames. If you're not sure, take your stem off and measure the diameter. I've also though about changing the fork on my Trek 520, but the 1" fork limits my options to virtually zero when trying to match the other dimensions of the fork, so I gave up on the idea. With a 1 1/8" fork, there may be some alternatives, but you need to not only match the rake, but also match axle to crown length.
Be aware that a compact crankset doesn't have a good intermediate-size chainring that is good for cruising along in with lots of range for speed adjustment on the back - it just has one ring that is a bit too big and one that is a bit too small for general riding. If I don't have a ring that is in the 38 to 46 range then I get frustated because I'm always double-shifting and/or cross-chaining.
Also be aware that the 11-28 cassette has some big jumps in it, particularly the 19-22 gap. I would opt for something with a little less range to keep the gaps smaller. To keep the same lowest gear, BBB and IRD make 12-28 cassettes that are way better than the 11-28 in this respect. If the high gear (11 tooth) is more of a priority, then get a SRAM 11-26.