That's going to be tough. If I recall correctly, it's either a 3mm or 4mm screw. If it is broken off flush, you'll need to drill it out, but that comes with a host of challenges and risks.
If it was me, I'd just get a case for the radio and forget the clip. Drilling out a screw that small will likely result in the threads getting jacked up. I'm not sure they make Helicoils that small.
Options:
Get a new chassis for the radio and swap things over (probably cheaper to buy a new radio)
Install the belt clip with the one good screw and some epoxy on the other side.