The M42 flange distance is longer than a Leica's, so an adaptor that takes that into account is theoretically possible, and could provide a full focusing range. (as opposed to the opposite which would only yield good focus for close distances). But, as noted, M42 lenses don't have the coupler for the rangefinder arm. It might be possible to mod a lens, but it probably would no longer be usable on an SLR. There are a ton of mount adaptors on ebay being made in China, I've never had a reason to look, but an M42 to LTM adaptor may exist, but it's unlikely to include rangefinder coupling.