the strongest way to combine both of the cranks is too thread both cranks put a stud in one side of the crank and wind it in with the other crank and get the right crank timing and weld it with a welder with good penetration attaching the case is another issue though if the case is metal you could just weld it if its aluminum your out of luck
If you saw the pic with the screwdriver I’m pointing at the nub where a bolt could go needs notched and drilled from the rounded side and it would be an opposed twin then. The regular replacement gasket will go right between the 2 crankcases.
I got rid of those weedeater motors were taking up too much space. I think the best way is to take diagrams from an exsisting snowmobile motor and scale it down. oliver motorized just built a 65mph bike with a snowmobile engine btw.
wouldn't it be funner to mount multiple CGs- front, frame and rear? hell you could do 3 or 4 on a trike axle,,, custom chrome pipes and it'd look ungodly lol