You need a 1" threadless fork for a Pista. Threadless forks generally come in one length and are cut to size so unlike threaded forks, you don't need to worry about the length/size.

The hardest thing for you is going to be finding a threadless fork that matches your current fork rake. Most forks are designed for road bikes which have a more relaxed geometry than track bikes. The Pista (well the 06) model has a fork rake of 28mm. Most road forks have rakes around 40-45mm.

If you currently ride with a brake then you'll need to find a drilled fork, which for a track fork is harder to find. I know Planet-X, Terry Dolan, Look, Alpina etc all make track specific carbon forks. The Wound Up fork is also another good option (See http://www.advancedcomposites.com/woundup.htm)

The other thing that complicates this is many manufacturers have moved away from the 1" to 1 1/8" threadless systems so your choice of fork may be limited further.

I would suggest asking this in the Fixed Gear/Single speed forum as there are a ton of Pista owners there and people can probably make specific suggestions about makes/models and price and their experiences with them.