Depends on your needs, but I'd consider having the original fork repaired by a good frame builder or a new set of blades put on with the correct dimensions. That is a feasible repair, but probably not fast or cheap.

What's basic problem when you install the new fork? Can you show some pictures? If the blades are too short, you won't be able to clamp in ANY 700c wheel. If they're just a little too short, you might be limited to 700x23 rather than 700x30 or so. Neither of these problems will get better if you space up the crown race to head tube interface.

The only way I can see a 700c wheel fitting in the fork but needing to raise the head tube (top tube tilted down?) is if the new fork has a lot more rake than the Trek fork did, which means you did not buy the right fork. You should be able to find the original rake of your 900 fork by looking up the original brochure for your bike, on www.vintage-trek.com.

My main thought is it sounds difficult to modify an existing headset to get 15 mm of spacing between the crown race seat and the bottom of the head tube, and very unusual for someone to have designed and manufactured such a product in the past. I could barely imagine a headset with 2 or 3 millimeters difference in lower stack height from another one. Bigger bearings might get you a millimeter or so lift. Putting a spacer under the crown race takes the crown race away from the area where it's intended ot have an interference fit, and introduces flex into the steer tube at a very highly stressed area. Putting spacers under the fixed cup reduces the contact the fixed cup has inside the head tube, also compromising strength at a critical point. A millimeter or so at this point might not make much difference in integrity, but it also doesn't get you close to 15 mm extra.

Need pictures, the problem doesn't fully make sense yet.

Road Fan