The Harbour Freight engines you mentioned are all too big. The Greyhound is a 2.5 hsp motor. Yes, it is slow revving so what makes it work as a bike motor is a suitable transmission. I use the one from EZMotors called a Qmatic. It turns an industrial motor into a smooth running motorbike engine with plenty of low end torque to take you from a dead stop right up to too fast. Jim Davis has had his up into the 40's and he's a big fellow. I don't use a speedometer and have no interest in going 40 on a bicycle, but I do appreciate being able to climb hills without a problem. Quenton Guenther at EZMotors re-calibrates the clutch in the Qmatic if it is for a Greyhound, making it engage sooner to accommodate the lower revs.
That's the link to a build I just finished which will give you an idea of what's involved. I'm still doing the build thread. The pictures of it done are here in this thread...
Between the two they should give you a better idea whether or not this engine is for you. Good luck. Hope this helps.