First question: is it a 1" (nominal, 7/8" actual OD of steerer tube) or a 1 1/8" (nominal, 1" actual OD of steerer tube) ?

Replacing headsets is pretty trivial. I made a headset press with a length of 1/2-13 threaded rod, an assortment of washers, two sets of spherical washers (to prevent side loading of the cups), and two large knobs threaded 1/2-13 (Amazon has everything, Home Depot has everything too). Knock out the old cups with a section of ¾" copper pipe or hard wood dowel and hammer.; clean up frame, very lightly grease the frame and press in the new headset (both ends at the same time, or on at a time).

If it is a 1" threaded, get a long one, and cut it to length, clean up, take to a local bike shop and have them extend the threads. Call around first to confirm the shop has a die to do the threading, they may also want to cut it for you to ensure a clean square cut. Fixture for cutting is highly recommended. You can find a die on eBay and other sources if you want to DIY.

If it is a 1 1./8" threaded, call around, you may find a shop with a die. I couldn't find a shop with a die, nor could I find a source for a die. I switched to threadless.