This may be a little off topic, but the question of engine mods to boost low rpm torque comes up again and again.
This quote is in relation to Honda reworking an existing engine design from a sports/tourer (Xelvis, VT250) to do service in a cruiser (VT250C, Magna 250).
"The engine came from the basic Xelvis which is a well proven 10,000rpm motor. To achieve the right feel and sound, the crankshaft was altered, a bigger flywheel used, longer gearbox ratios and new valves and springs substituted for old, and, most importantly, a totally different carb setting utilised. The torque curve is incredibly flat for such a small engine."
Honda VF250 V
As shown above, significantly changing the torque characteristics of an engine is possible, but it sounds like a lot of work.