Idk, did you change the plugs when it stopped running right? Also, did you put new fuel lines on the machine and including the intake line? What about the connection from inside to outside the tank, is that clear? I would get a cheap honda clone for it. Not even the 16hp, just a 13 or something. Harbor freight sells the cheap as does ebay. Your amphicat is light and the tranny does not rob that much power. A smaller engine should work just fine in there. I think a 16 or 18 vanguard would be over kill. Remember, the rpm of an engine determains top speed so as long as the engine has enough umph to move it, all engines that run at 3600rpm will generate the same top speed.
l like to buy stuff and no I don't do payments!