1. Did your Mystic have the video hack done to it or the ResEdit hack?

2. Will it boot from the (free) 7.5 disk utilities floppy?

My experience of the Mystic CC is that System software installation on it is generally a painful fiddle, so I would not give up on the replacement HD just yet.... However, if the machine previously ran with the stock video resolution of a CC, then you will need a stock CC logic board to get any further, as far as I know.