Hi Graham
Have you done the deed and bought it? I have a 640 pilothouse like yours, Im based at ribs marine Christchurch. Have you spoken with Simon there who is the main agent for Quicksilver.
By the way like you I have toyed with changing the engine, I have a 115 mariner 4 stroke.
I have done a lot of research and the best weight to power ratio is the yamaha by far, on average the yam is 40 kilos lighter which is huge on the handling and fuel consumption and a quieter engine with better security and user dials. By the way your boat is only recommended for 115hp 85kw so you would have to notify your insurance.