You are looking for Maccel, version 3 is the last version I believe. It is a simple 2x accelerator, but it works well with the original Atari mouse and actually makes it useable..
It is on a disk image on this site.. sure it must be other places
Don't worry it is for Falcon but the program will work on ST, although you may need to use an emulator to extract it.
If you want something a bit more souped up, Silkmouse is an alternative (I believe it also has a CPX). I never got on with it, but it does have some more advance features..