You seem very concerned about making it to top heavy. That being the case have you compared the weight of the average recomended .40 size engine to the .75 you are now concidering? In an airframe that is designed for a .40 size engine we can usually get away with going for a .50-.55 size engine without to much difficulty. A .75 would put a plane way out of ballance and require motor mount modifications. I understand price is a concern but I would think reliability would be as well. I would not put anything on the water that I could not trust to stay running and in this hobby... you get what you pay for. OS .55 AX would be my choice. I have one in a Funtana 40s and it has never dead sticked in on me.