While a brushed motor (that one is series wound) is typically "easy to use", this particular motor may be pretty disappointing:
Speeds Up to 20 mph
3.5 HP @ 3,000 RPM at 36-volts
4.7 HP @ 4,200 RPM at 48-volt
At 750w per HP, that's only about 3500W. I'm sure it will take bursts of way higher power, but brushed motors are usually a lot less efficient than brushless, *and* the higher voltage and current you push thru the brushes, the more power is wasted there as heat, and the faster the brushes wear out.
I had a little more power than that in 2 ebike hubmotors on Crazybike2, and it only did about 4 seconds to 20MPH, at around 400lbs total including me on it.
For a realistic offroad motorcycle conversion, you're probably going to want 8-10kw at least, especially if it's a larger / heavier bike (and/or rider), or you have any kind of slopes or rough / draggy surfaces, etc.
One advantage the brushed motors do have is that they can have very high torque at startup without positioning issues, vs typical 3-hall type brushless motors in the same situation (other position sensors that are continuous-position output and controllers that use this to their advantage can be equivalent to brushed startup torque, however, and more efficient...but more expensive and harder to setup / program).
Since you don't need fast speed, you might get away with a good gearbox (or high ratio on the drive chain to the wheel) to gear down the motor a lot, and get a pretty torquey setup
What are the specific usage scenarios and needs for this project?
Range?
Riding conditions? (wind, terrain, hills, etc)
Speed (max and cruise)?
Total expected weight (incl rider/cargo/etc)?
Budget?
Any other details you can provide may help us help you figure out what it will really take to do what you want it to do.
Regarding adapting the motor, you can find a compatible splined shaft at a junkyard (bring the motor, and test fit it on various ends
) if you can't buy one from that site or one of the others that sells the motors with the same spline. Leave enough of the shaft when you cut it off it's source so that you can install a sprocket adapter ring for your chainline, and perhaps a pillow block bearing on the far end to mount to the frame to keep stress off the motor bearings and such.
Alternately if the clubcar gearbox is small and light enough you could just use that for the geardown, if it fits in the frame with the motor (and battery). Depending on the gearbox's output, you might end up mounting the motor lengthwise instead of transversely, if the gearbox's output is at right angles to it's input.
