If you have a leak from the screws being loose or stripped...you can rotate your existing metal valve(after lifting it out) so as to drill new screw holes between the old ones. A little aqua Seal, or silicon, would help insure a seal. Of coarse, be careful not to drill too far. Just to go through a metal plat approx 1/8" - 1/4" down. I just did this procedure last week. Worked well.
If you have to replace the whole valve assembly, including the "boot"...that's a whole different story and could get very involved.