I have the same problem with my old HR valves. You can buy just the top half of the valve through various sellers online, which means you don't have to cut the old valves out. The top half will screw right into the bottom half of the valve.
If you want to get a few more trips out of them before replacing.... Try unscrewing them and cleaning the valve of the grit and grime. Sometimes that will help. Also, using a bit of vasaline in the valve can sometimes help stop the leak if it is small. However, this makes for a bit of a mess. It did work for me for part of last season.