If you are interested in the 50 Summilux ASPH, I would imagine that you want to have a lens that is fast and sharp--specifically, sharp when wide open at f1.4. To me, I don't think you can compare the 50/1.4 ASPH to the 50 Summicron because while the latter is reputed to be sharp, it can't achieve f1.4. Basically, I wouldn't consider the 50 Summicron an "alternative" to the 50/1.4 ASPH because for me f2.0 is no alternative when I need f1.4. If anything, the 50/1.4 ASPH is an alternative to the 50 Summicron because it is sharp at f2.0 but can also go to f1.4.
From what I have seen, the Nikon 50/1.4 Millennium does seem to be fairly close, but it seems to be hard to find those lenses readily and it seems like a hassle to get the adapter for M mount. The Zeiss 50 C Sonnar seems nice, but it's a Sonnar design with the typical Sonnar signature (which may or may not be to your liking, but is certainly different from the 50/1.4 ASPH signature). The Voigtlander 50 Nokton is also fast, but I don't know too much about its signature and whether it is particularly sharp wide open.
If you are only looking for a Leica alternative, the 50 pre-asph Summilux E46 is a great lens that goes to f1.4 but is not known for being extremely sharp wide open. Personally, the 50 pre-asph Summilux E46 is sharp enough for me. While a small number of people claim they don't see that much difference between the pre-asph and the ASPH, it seems that the majority of people feel that the ASPH is significantly sharper wide open. At this point, there doesn't seem to be a readily available alternative that is nearly equivalent to the ASPH, but there are a number of choices that require some degree of compromise.