see what size forks you have. Probably 38mm or 41mm that year. Find a later model bike with same diameter forks and disk brakes. Make sure length is similar or it will hurt your handling. If axle size on new forks is different than what you now have, you may have to use wheel from the bike you are getting forks from. And you may have to find/fabricate different wheel spacers if distance between forks on your current triple clamp is different than the donor bike. It can be a pretty involved affair. Personally, I'd just leave it as is and enjoy it for what it is: a decent vintage MX bike (that someone who collects vintage bikes may be interested in buying and letting you get a newer model with disk brake).
I have a question for you...why would you want to spend a large amount of money swapping out the front end of a bike that is worth nothing? No offense, but that is a $600-$700 bike on Craigslist in good condition. You are going to end up with a $600 bike when you are done spending hundreds. Some vintage buyers might even find it less appealing because it is not original. Dont waste your money....if you can get it running with a throttle cable, shift lever, and brake pads..I say go for it. However, if you want a later model bike with disc brakes, water cooled, etc..go buy one on Craigslist.