Start by replacing the seat (but save me the old one). You might need a new seat post to fit the modern seat (save me the old one). You might need a new frame to fit the modern seat post(but save me the old one). You might need a new crank to fit the modern frame (but save me the old one). It should come with a fork (so save me the old one).You will probably need new wheels to fit the modern frame (but save me the old ones). You might also have to replace the stem and handlebar if they don't fit the modern fork (but save me the old one). There might be one or two parts that still don't fit; no worries, you can replace those, too (but save me the old ones).