I have attached two photos of one of our tandems – a Sun “Brikell” tandem that appears to have a frame identical to yours. As you can see, I have modified the captain’s seat post in order to bring the nose of the saddle over the bottom bracket. This provides me with a great deal more control of the bike at very slow speeds. The somewhat “feet forward” design of this frame appears to be intended to give both riders the ability to place both feet on the ground while seated on their saddles, but it makes the bike handle in a dangerously cumbersome manner at low speeds and is extremely inefficient for delivering power to the pedals, hence seat post mod and the electric motor.
In the second photo, you can see that I have replaced the rear wheel with one mounting a much more robust tire and a direct drive hub motor. As you see, the 48V battery is mounted in a carrier in the luggage rack. Mounting the battery in this manner will, in theory, create handling issues, but hasn’t presented any problems in our experience. Because the bike is a tandem – a heavy and cumbersome one at that, we don’t find the high center of gravity to be any more of an issue with the battery installed than we did before the bike was electrified. Not surprisingly, we enjoy our rides far more with the electric assist. Riding the tandem seems more like fun and less like work these days.
I have installed hub drive kits provided by Grin Technologies, E-Bike Kit, and Ebikeling and mid drive kits sold by Electrify Bike. I have found all of these vendors to be most helpful in supporting their products.