Re: what did you do to your motorized bicycle today?
Got any pictures of the trike?
I know most of them are of the step over frame type so there's no top bar to mount the tank in the normal way.... Mounting the engine on the other hand will be the easiest thing to do.
At the rear axle you could run a separate sprocket for the engine or you could use the center shaft where the pedal chain goes before a second chain goes to the wheels, you could put the 44 tooth sprocket on that shaft and a freewheel on the other side so it could still be pedaled but won't move the pedals once the engine takes over.
For starting the engine, pull starting is very effective and these engines can easily start on the second or third pull even in the dead of winter once the engine has been run a few times to loosen it's self up some.