You can't substitute a 8 HP rated gas engine in an application that requries an 8 HP rated electric motor. You could easily substitute an 8 HP electric motor (or smaller) for an 8 HP IC engine. My 3.5 HP rated electric lawnmower easily outperforms my old 5 HP gas mower. A 3/4 HP electric motor that runs my little milling machine does work easily that a 3/4 HP rated gas engine couldn't do at all if I tried to substitute it. The link made it pretty clear that IC's are rated in peak HP and electrics are rated continuous HP. Torque is maximum at lowest RPM on electrics as well which makes a gas IC substitute another problem (meaning you need a torque amplification device of some type, a transmission, to get something started with an IC that will start without one using an electric (another attractive feature of an electric vehicle power train). This increases the needed rated HP of a gas engine another notch in a substitute for an electric. You don't have to believe me, just try it and convince yourself. I know from experience, I've designed and built many, many different types of mechanical things (it's a hobby currently and I worked in mechanical/electrical mainteneance in Insustrial plants many years ago). You may need to learn the same way if you don't trust what the pro's say or what I have learned from experience.
Depends on the application. The main thing about an electric is that when you load it and decrease the rpm, the torque doesn't go away. You can load the thing until it stops and it doesn't die. IC engines necessarily have peaky torque curves.
However, the advantages of IC are many. The main one being that it is far, far easier to store/replace/refill your energy source when an extension cord is out of the question.