Only if the author has been acknowledged as an expert by reliable books, newspapers and magazines. Otherwise it doesn't meet Wikipedia's policy of Identifying reliable sources. For example, Kevin Ash is a journalist and published author, and so his personal site on motorcycles is considered an acceptable source for some types of information. I don't see what the problem is. Why can't people just go to 1968k0.com instead of Wikipedia for this information? I can think of many other similar personal web sites that focus on a single model of motorcycle which also don't meet Wikipedia's standards, but that doesn't prevent people from getting the information.