The following is most likely TMI, but hopefully it will clarify things a little.
There is a HuaSheng 142F, also known as a HS142.
http://www.bicycle-engines.com/49cc-Gas-Powered-4-Stroke-Engine-Five-Eighths-Inch-Shaft.html
There is no HF142.
The HS142 is 49cc, has 2.5 horsepower, peaks at 6800 RPM and redlines at 8000 RPM.
Then there is the HF79.
http://www.harborfreight.com/engine...cc-ohv-horizontal-shaft-gas-engine-69733.html
The HF79 is 79cc, has 3 horsepower, peaks at 2500 RPM and redlines at 3600 RPM.
Both can get you moving fast down the road, they just need different gearing to do it.
The HF79 has more torque so in most gearing situations it will climb hills easier and be quicker off the line.
Now, if only I could mod a HF79 to redline at 8000 RPM!