Assuming your the rear spacing on your new frame will be 130mm, Harris has a selection of hubs which fit this application, fixed on one side, and freewheel (single to 5, 6, or 7 speed) on the other side. Probably your best choice is a Phil Wood hub. This way you can build one wheel, have two gearing possibilities. Of course the switch will take a little time as you install the new chain, derailleurs, shifters, cables, etc. Best of luck on your project.
Scroll down the link and you will see so many variations (i.e. spacing, spoke holes, cog and freewheel choices, etc.).
Either way will work. If the axle is long enough on a 120mm spaced hub you can add spacers, or if the axle isn't long enough you can swap it out to a longer one and add spacers.
Alternatively you can just buy a hub with the proper spacing. If you are on a budget, Formula is a good choice, if you aren't worried about cost Phil Wood are wonderful hubs. I have both, and would buy either again.
There are a few fixed, free, and fixed/free hubs available for frames with 130 mm and 135 mm rear axle spacing. It's probably better to buy one of these hubs rather than add spacers, if only because you might need a longer axle in addition to the spacers on a stock 120 mm hub.
You should also consider the chainline when selecting your hub. Generally, track chainlines are 42 mm. If you want to switch to gears without having to replace your bottom bracket and crankset, you might want to go with a wider chainline.
Phil Wood, Paul, White Industries, and Harris Cyclery sell hubs for 130 mm. White and Paul make single speed (not fixed) hubs for 135 mm spacing. Phil and Harris have 42 mm chainline, Paul is an unusual 44 mm, and White is 47.5 mm (which can be used with a standard road double). If your geared frame has vertical dropouts, the White ENO hub is a good bet. I have two and I think they're great.
See Sheldon Brown's glossary entry on chainline for more info on that subject.
"When man first set woman on two wheels with a pair of pedals, did he know, I wonder, that he had rent the veil of the harem in twain? A woman on a bicycle has all the world before her where to choose; she can go where she will, no man hindering." The Typewriter Girl, 1899.
"Every so often a bird gets up and flies some place it's drawn to. I don't suppose it could tell you why, but it does it anyway." Ian Hibell, 1934-2008
Thanks for the replies. I can go back to dreaming now. The next thing is to find the right Bullhorns - I don't want straight bars, I want ones with a little bit of pull back (similar to the Nitto Noodle) and rather wide (most are only 42cm where I want something like 46).
I had a good bike ... so I FIXED it
On track hubs, Velocity market a cartridge bearing model that is, I think, a rebadged Formula. It is available, I believe, in both one-sided, and flipflop versions. I source stuff like that from Abbotsford Cycles, in Richmond, Melbourne (they have a website and are "old-school" in their approach -- like touring, randonnees, and wool jerseys). The two hubs I have had are nicely machined and finished, and spin superbly well.
One note of very real caution, though, irrespective of the brand of hub. The first hub I bought, the one-sided track hub, I stripped the threads quite rapidly. I put up a question on the Bicycle Mechanics forum, and was informed that it was caused by a cheap pressed steel track cog, and that I hadn't cinched it and the lockring down near tight enough.
Amazingly perceptive, because that was the exact story, which I hadn't really identified in the posting. This all happened, by the way, around 20km from home, so it was a long walk with two panniers worth of grocery shopping!
Evidently, the pressed track hubs are uneven on the face that butts up against the hub, and the threads aren't particularly brilliant either. The result is the cog undoing and moving back and forth on the threads, slowly but surely shredding them.
So, my hard-earned advice is to budget for a good quality hub, a good quality lock ring, and at least a machined Dura-Ace cog. Since going this route with the second hub, I have not had one iota of trouble.
As to width, I used an old Shogun lugged steel frame that I recovered from a tip, so the dropout width was already idealled suited to the 120mm width. The Velocity/Formula hubs have a hollow axle for a quick release. As already stated, check your external dropout width so the axle can use track nuts (assuming you are going that route rather than QR).
My first FG project was a bodgy job involving an old steel road hub with the lock cog from an old Shimano cogset held on with a BB lockring. The lot went on a Peugeot lugged steel frame (another tip bike that I don't even have a model reference for) with cut-down and inverted drop bars, and the crappy Alex wheels that were the OEM on my Fuji Touring. The front ring was a Biopace job -- I kid you not -- and I had a ball with that bike around Hobart for almost 900km before I left for greener pastures.
Dream. Dare. Do.
Oh, and keep and eye on Australian eBay. There are bullhorn/cowhorn bars come up regularly in different styles. That's where I sourced both mine. It's useful to search for both cowhorn and bullhorn.
Dream. Dare. Do.