There are companies which service forks (like the Shock Treatment Center at QBP). Chances are your local bike shop has an account with QBP and might be able to arrange something to get it fixed, provided QBP services Headshok forks (if not, another non-Cannondale company might). Of course, the reason why your fork went bad may have something to do with servicing it / preventing it from going bad further, and given how old this bike is, they might (probably) won't service this fork. But you might find a different company that does service old Headshok forks.
As for replacing a Headshok (or Lefty for that matter, I think), you'll need a Problem Solvers Headtube Reducer for Headshok forks which will convert Headshok to 1 1/8". You can read the instructions, but basically, you pull out the Headshok, remove the bearings, press in the reducer and then install your headset and fork. Other manufacturers also make parts for converting from Headshok to 1 1/8" (e.g. Cane Creek's Double Xc).
You'll need a shop to install the Problem Solvers reducer (or other manufacturer's options), since it requires a bearing press (which home mechanics don't have; its about a 150 dollar tool that you will probably use once in your life). So, while they're at it, they should be able to set you up with a compatible fork (by measuring axle to crown and what your other needs are) and everything else.
All that being said, this bike is close to 20 years old (maybe a bit more, maybe a bit less) -- the parts you need and labor will likely make it more favorable to buy a different bike; before the fork, you may be spending 50-100 bucks on the parts to get a standard fork to fit in the first place. I'd probably bet that a basic hard tail today would be better across the board than a 20 year old full suspension bike, too.