How would mount 80cc engine on this?

you may have too mount the engine off center toward the left side using brackets and mounts or raise and tilt engine to clear chain. if youre lucky the bolts for the sprockets will slide threw the spokes at the right places.you could drill and mount sprocket to wheel hub. or mount disc brake to wheel and mount to that. or buy hub kit for rear wheel which includes mount for sprocket. or i know you dont want to replace rear wheel because you like the style spokes and wheels,but that is another option. im not sure if welding is right for the wheel as it may warp the rim,plus if the wheel goes bad youll have to replace the sprocket plus the wheel.but if youre good at tac welding go right ahead and while youre at it weld some engine mounts so you can offset the engine.
and is this bike frame steel or aluminum,make sure you know how to weld it.goodluck.

Like this... I found it best to reduce the bicycle chain ring to 36T. It will able you to lower the motor.
http://motorbicycling.com/f15/redbs-greenline-s-t-r-t-3991.html