By the time I got back into biking last year, the Aquila's seat post was permanently seized and it's shifters were shot. I converted it to a single speed with a chain tensioner and put on studded tires for winter. It was a beast, but the more I got into riding my fixed gear road bikes, the more I wished that the Aquila had horizontal dropouts so that it could be fixed, too.
I remember that the late great Sheldon Brown wrote that a fixed gear mountain bike would be the ultimate winter ride, so I started looking for an early version from the mid-eighties that would have horizontal dropouts. (Looking again for the quote today, it seems that Sheldon said no such thing. He may have thought it, tho...?)
Last summer, I finally found my bike, a 1987 Schwinn Mirada in flamingo pink--a mid-range bike from Schwinn's early Mountain Bike lineup. It's got a CroMo frame, but it's still heavy as a tank. 26" wheels, no suspension to crap out--perfect for my purposes. It cost me $40.
With a vintage road bike, fixed gear conversions are pretty easy because you can always buy a 700c flip flop track wheels and pop them in. All you've got to worry about is chainline and maybe the reach of the brakes. But with 26" wheels, it seems that fixed gear wheels simply don't exist. I finally found one on some obscure Google store, but when it arrived it turned out to be 650b, the wrong size altogether. So, I took that wheel apart and relaced the hub to an old Weinmann "Great Wall" rim.
I added a new stem, fixie flat bars, and studded tires. At first, I thought the old Schwinn canti brakes would do the trick, but they turned out to be super crappy. I fussed with them forever but they never stayed in alignment and were prone to failure at critical moments. So, the Mirada now has new Tektro v-brakes. We're good to go.
Then it snowed! I can tell you that the Mirada is a great ride on packed ice and snow. It's a surefooted beast. No one would say that the Mirada was ever a top of the line ride, but as a winter beater battling snow, ice, and salt it really is the best. Honestly, Sheldon was right, even though he didn't say it--a fixed gear mountain bike really is the ultimate winter ride.