First, what is the goal of upgrading to a 1000 watt motor? Next, what motor do you have currently? You will need to determine if the dropouts can handle the additional power, assuming you currently have a hub motor. A picture or link to the bike you wish to "upgrade" would be useful, then I wouldn't have to make so many guesses.
You will probably, though not certainly, need a new controller. Changing just the stator will almost certainly not work.
You have a cargo trailer. You are hauling gas to a cabin cruiser which has a 14-foot dinghy. I can assume you are talking about more than a couple 5-gallon cans, likely a 20-40 gallon tank. That is a lot of weight. More info would be useful.
Parts for the BIKE will come from whoever made it, or standard bicycle parts if feasible. Parts for the Bafang motor will come from Bafang, controllers and displays available from many vendors.
Putting the battery on the trailer is somewhat feasible, however the controller phase wires are ideally as short as possible. Doing these also means the bike is not usable without the trailer.
Putting a seperate motor, controller, and battery on the trailer itself might be a solution. Hauling gas, some care will need to be taken with anti-spark measures.