I feel bad you are having engine trouble, sounds like maybe a leak-down test is in order for your headgasket situation. But don't be hatin' because everyone is out enjoying the weekend on there watercraft and not able to help. I have reason to believe that I am the only man in ALL of West Michigan NOT enjoying the evening at Coast Guard festival due to my commitment to bailing 1270 bails of hay!
I am not one of the experts, but I am the mechanic of my own boats and my friends. A little more tech. information would not hurt. One question I have is how hot did the motor get? Was this a severe overheat, or only mild? Did you change the oil in the motor, and if so, did you find water in it and how much? Most likely you have boiled the viscosity out of it and the oil has substantially less ability to lubricate and needs to be replaced. It also needs to be inspected for the presensce of water and quantity of it. If the riser gasket is leaking, it must be replaced. If the overheat was severe, the manifolds and risers should be inspectd for leaks and cracks also.
As for the poor idle quality, and throttle response, more information will be required before an "edjucated" guess can be given!
P.S.- Try and be one of the green emoticons, and not a red one!