you need to get a little gear that probably broke inside the cluster...i think you can get it from ford. take out your cluster and see if you can see if its really broken or not.
you could just not fix it til you are about to sell the car.... > jk, i bought my v6 with the odometer broken, but i never haad the need to fix it. looks like im no help
I would be honest with myself and get a new gear as soon as possible, yes it may decrease your resale value at this point, but if I remember correctly you already have a second motor in it which kinda makes the miles on the car LESS significant. idk, just my idea.