I have a 1200w ust portable generator and when I went to start it the cord it the cord snapped. What tools do I need and how do I replace it?

You do not need any special tools but you do need a third hand somtimes to complete the job.
Here are the basic steps after you remove the starter from the engine:
Prepair the new starter rope by burning the end to keeping it from fraying and shape it into a point. Use a rag to shape the point because it will be very hot. (about 5 feet of good qualty starter rope size to match existing. It is sold in sizes form about 3.5 to 6.5) Tie a temp knot on the other end of the new rope. Remove the old rope and align the hole in the spool with the hole on the starter cover and practice incerting the rope thru the guide hole on the cover into hole on the spool . This will be the final action after you wind the spool. You should be able to hold the spool and insert the rope thru both holes and tie a knot while holding the spool in a wound state. Once you have mastered incerting the rope thru both holes and tying a knot you are ready to begin.
Wind the spool in the same direction that the spool turns when you pull the rope. You must wind in the correct direction or you may bend the rewind spring out of shape. The spool will begin to tighten as you wind and will eventually stop turning when the spring is fully wound. Once fully wound unwind spool one full turn and stop when hole on spool aligns with hole in cover. Now insert rope as practiced thru both holes and tie knot with free hand or have helping hand. Once knot is tied slowly let spool take rope in untill no more will go in or you reach temp knot. Now tie pull handle and you are done. Good luck