You would need to create your own repositories of essentially all checked out frameworks, make changes in LoopKit, and then make sure that cartfiles for all frameworks point to your own frameworks. It’s a bit tedious, but not too difficult. Start by cloning jojo Loop or whatever branch you are using. Then look through the cartfile and clone all dependent frameworks that are different from standard Loop. Then make appropriate edits in your LoopKit and cartfile’s of Loop, as well as carfiles of all other relevant dependencies. For such small changes, you can do all that on github from a browser, no need to use Xcode at all. In the end, you download and install your Loop as usual. The best guide is the jojo branch itself - just do whatever Katie had to do to create a branch that points to her private version of LoopKit.
The fix only changes the override for 0%, to avoid division by zero. The part of the code: 1.0 / ($0 == 0 ? 1.0 : $0) means: divide by 1.0 if the percent-override is 0, otherwise divide by the percent-override. So, you’ll have a very special zero % override that produces ZB, but keeps ISF and CR untouched. If you wish to have some other ISF or CR scale factor for 0 override, just change 1.0 to whatever number you like, e.g. 1.0 / ($0 == 0 ? 0.5 : $0) would bump ISF by a factor of 2. Makes sense?