Search “trigger potentiometer”, you can find them by the bunch.
Although it is going to be a difficult repair, you have to desolder it from the flimsy controller board, which often gets overbaked and traces will fall off the board. Also you have to mind not to melt other plastic parts.
If you can reach them, cut the leads off the potentiometer, disassemble the plastic parts, clean the leftover leads from the board and solder new one on.