Measure the existing forestay.
Second that Emotion, as the song sez.
With broken strands, the existing stay needs to come down. Lead the spinn. halyard to a solid part of the bow and sock it up tight. If you feel that a back up is needed just bring the jib halyard down to a separate place near the bow and tighten it up, too.
You will need to go up there and undo the fore stay and lower it down with some line. Gonna need a helper or two for that. Once it's on the dock you can take it apart.
How old is that fore stay? Age of furler? If the number "20" or more comes before the word "years" it's time to change out the standing rig as well (unless the rest of the standing rig is newer).
If you have some digi pix please attach 'em.
Best of luck,
Loren