I don't know what the correct torque value is, but I do know that the flywheel nut is approximately the same size....or at least I think it is. The flywheel nut on your motor is 90 Ft LBS.
When you say "it won't reverse" are you saying the motor won't go into reverse, or the nut won't come off?
If the motor won't go into reverse, then you have a different problem. The nut has nothing to do with the ability to go from reverse to neutral or forward unless maybe possibly you forgot to install the washer on the prop shaft before you slid the propeller on the shaft.
If the nut won't come off, then get a bigger wrench. 55 FT LBS should be easy to get off.