The problem I see is that there is a big difference in the head tube angle between your 520 and the HTA of modern bikes that modern forks are built for that I'm aware of. If you are trying to keep the handling characteristics about the same with a new fork then you need to keep the steering trail about the same. Trail is a function of head tube angle and rake offset. A change in the length of a fork will effectively change the HTA. The HTA for your bike is 71 degrees according the the chart. Most road bikes these days have HTA's around 73 to 74. Modern Treks are some of the steeper HTA's. If I'm reading the chart correctly your fork has a 52 mm rake offset and a trail of 63 mm. The relatively high rake offset is to help offset the layed back HTA to result in a very reasonable 63 mm of trail (more than Trek's modern "racing" geometry).
What I'm getting at is that typical modern replacement forks are designed to work with steeper head tube angles. But it may be possible to find the correct length and rake offset. I don't know. It is the 63 mm of steering trail that you should try to duplicate or at least get close. Assuming an identical length A rake offset of 50 mm would increase the trail and might make the bike turn a little slower, not necessarily a bad thing.
Here is a site with some useful information including the formula for calculating trail: http://www.bookrags.com/wiki/Bicycle...cycle_geometry
The handy calculator that I use to use is no longer available.