Honda-SOHC

Hi. I'm a newbie so hope I do this correctly! I have just purchased a 1977 CB750F1 as a restoration project. When trying to set the timing I noticed that one of the lugs for the points backplate screw was broken. It's integral with the lower crankcase. I could try to get a repair done but was thinking it might be cheaper to get a used lower crankcase. There are quite a few out there but they are K series or F2's. I was wondering what, if any, differences there are between the F1 lower crankcase and say a K6 or F2? I know the F1 was the first to have a rear disc brake and someone said this meant a slight change in the case, but I don't know. Anyone able to enlighten me?