If the marathon distance of 42 km is important to you then there are a few races of that distance in Nepal. With big hills and high altitudes 42 km feels like an ultra and is a sensible and challenging distance to cover in a day. So it is a great distance for race organisers to arrange as all runners can be over the finish line before nightfall.
Here is the list of trail marathons and trail half-marathons that we know about. If you know of more that should be listed, please email email@example.com