Swapping out the original forks on my Trek 990 really gives new life to the bike. Last night I took the Trek 990 for a spin after swapping out the original front suspension, headset and stem. Wow! I love this bike. The original Trek DS2 fork was an early air fork with limited travel and almost no adjust-ability. It was good at the time, but times have changed and the seals were starting to go. I wanted to upgrade it with a modern fork with more travel, but didn’t want to drift to far from the original design center and look. I also wanted to keep the existing vBrakes. I ended up finding a good deal on a Manitou R7 with VBrake bosses and 100mm travel. 
The DS2 is a threaded fork, swapping it out with the R7 also meant upgrading to a threadless headset and stem. I went with
The whole job cost about $400 and 2 hours of work. It was time and money well spent. I had never replaced a headset or cut a fork before but with advise from friends and The Big Blue Book it went smoothly. After the conversion, the bike climbs like a bunny on mate. The fork is very light and with the fork locked out is super stiff. On descents, the bike gives me more confidence than my SuperLight with its old Manitou Skareb fork. That fork will be the next one to go.
Robert Larson 
Hi, i have a 1991 Trek 990 with original rigid fork. Do you know if i can do the same upgrade of forks that you did, or if my frame is fundamentally different for some reason?
Thanks, mark
You should not have any problem replacing the fork with a new one. But it is difficult to find a quality fork that is still fitted for v-brakes. Almost all modern quality forks are disc only. I never updated this post but in the end I replaced the Manatou fork with an 80mm Magura Durin Race fork. The Manatou was a little “noodly” and at 100mm it was too long and threw off the frame geometry. 80 mm is better for that bike.
However, even the Durin is no longer made. I found a NOS one on eBay that has worked well.
Another alternative is to just bite the bullet and convert the bike to disc brakes at the same time. It will be a lot more expensive and you will need a conversion kit like this one from Brake Therapy. I like the Magura Durin with the 990, but if I had it to do over, I would do the brake conversion too.
So, my recommendation? If you love the frame as I do, swap it out with an 80mm XC fork and convert to disc brakes.
Thanks for the reply! We live near Minneapolis, which has a pretty active Craigslist, so I’ll be on the watch for an 80mm fork that fits the bill. In the process of transferring all of my early XTR stuff over from a slightly too small first generation S-Works (1″ threadless) frame that i built up 20 years ago. Regards, Mark