First, I think the only way to get the detailed information you are looking for, is to ask Briggs and Stratton. I do not think you are all that likely to find a mower enthusiast who has made a study of it.
Second, if you were to import an engine that is not sold locally, you will have to expect to import at least some of the spare parts for it (the ones that are not the same as the locally sold engine). Working out any warranty issues from long distance might also be rather slow.
Sometimes when there are frequent model changes, a low volume distributor (such as B&S here in Australia) may decide not to handle all of the models, but to wait for major changes each time. Otherwise they would constantly be liquidating old stock at a discount, and stocking spare parts would be a nightmare. I am not saying that is what has happened here, but it is a possibility you might be able to clarify with a phone call to B&S.
