Whip out a caliper and measure the wall thickness. Then measure some threaded forks, or ask around on BF what the recommended min, thickness is... Or find a good shop and have them do this part as well.

BTW, what you are about to do is a crime against cycling. WTF do you want to go back to an inferior design when the fork can do better? From the price of thread cutting you could get a threadless stem... If this is for track racing, def. go -less for stiffness.

Ló "quill stem hater" Farkas