Good question!
I think it depends on whether you want your best possible time for this race, or just a good time, and a bit more improvement for future races. The training you do this week can only adversely affect your race performance, especially so the more intense and close to the race itself of course.
Actually I would have thought that if you can do a 50 minute 10k in *training*, you should be aiming for more like 1:50 in a half marathon. You are probably a lot fitter than you give yourself credit for.
For speedwork you don't want anything too strenuous, so how about half a dozen or so 400s with plenty of recovery tomorrow? That should do some good for your future races, with a pretty minimal effect on the half on Sunday.
Good luck, please post after the race and let us know how you got on.