The K model stayed in production until 1969 as the KR.
The Sportster began manufacture in mid-1956 for the 1957 model year, traditionally announced in August or September of the prior year.
There are big improvements to the KR in 1954 and 55, but these can be retro-fit to earlier without surgery. An argument can be made that the 1957-67 KR was substantially unchanged, and offers no advantage compared to pre-56 and should be legal, but the 1968-69 were very different.
The 45" flathead "G" motor (mechanically equivalent to the earlier "W" = lowest performace model) continued production through 1973, but again there were no changes offering an advantage in the 1957-73 models.
My credentials? Shameless self-promotion:
http://www.victorylibrary.com/45PER.htm