Whenever I replace a gas engine with a non-stock one here's what I look at.
1) Will it fit? If the mounting bolts don't line up, can the machine it mounts to be drilled, or can an adapter plate be made? How about overall dimensions and operating controls? Shaft diameter, position relative to mounting bolts, height, etc. If it's belt drive, alot of this can be adjusted.
2) HP needs to be the same or more, and RPM must match closely. Almost all gas engines are standard rotation, so it'll likely turn the right way.
3) If it's pull-start, make sure you'll have easy access to the starter rope handle.
4) Oil fill and drain; occasionally, you'll need to add a short length of pipe to the oil drain, or changing oil will be a big mess. Remember to do this before you add oil. Depending on obstructions, gas tank fill location must be considered as well.
If all of the above are OK or can be modified, you'll be fine.
P.S. Torque is a function of HP vs. RPM. For example, 1HP at 1800 RPM=3 ft.lbs. of torque; 1HP at 3600 RPM= 1.5 ft.lbs. of torque. HP is the ability to do work; 1HP is the ability to lift 33,000 lbs. 1 foot in 1 minute. Torque is how hard a shaft is turned. You can't have HP when a machine is not moving, you can have torque whether it's moving or not.