What is it that you don't like about the bike that you would want to "improve?" Are you using the bike for just general all around bike riding or do you tour on it as well?
Bottom line is that the original parts are pretty good on that bike and I would be in no hurry to swap any of them out unless they are worn out or the bike isn't doing the job. The biggest "upgrade" on any bike, but particularly an old one, tends to be the wheels especially if they've seen some hard use over the years.