If your going to DIY a new sail cover, buy some high quality UV resistant thread. You'll probably get 20 years out of the sail cover that way. I did a double layer up near the mast portion of the cover and then a second layer at the end of the boom, seems these are the high wear areas.
I second the UV resistant thread. Also, as it's unlikely you can heat cut it as previously mentioned, you could potentially use a serger on the edge to prevent it from unraveling. You'd need some heavy duty needles though.