Use the stem calculator:

http://www.brightspoke.com/t/bike-stem-calculator.html

pay careful attention to the stem height and stem reach. Use your current stem as a starting point and compare it to the pista stem. Typically if you go lower, you're gonna want to go shorter if you want somewhat of a similar feel. Of course, you can't be 100% sure about how it will fit until your new stem is installed.