If you’re running a 36 try a 38 or 40T sprocket. The sport Athena will still get you close to 50 mph on a 40T. I don’t think any of these engines are gonna hold up running them WOT for long periods lots of times. My temps run a consistent 180-190 the way I ride.
Anything over 35-40 mph is way too fast for a motorized bicycle unless you’re competing in competition sports with all the protective gear to support you anyway.
My cast iron top end did have a good bit more low end torque over the sport Athena on a 38T but not so much that I felt the Athena was no good. I prefer the Athena but I cruise up to 25-30 mph gently and do not rip up fast a lot. It does run superb though for my style.
My Athena sport really doesn’t come to life for abrupt torque until around 7,500 rpm’s. It’ll jump from 40 to 50+ in a blink.