A few observations bro! One, you hoist will not lift the engine up enough to clear the engine bay because you have used a short length jack, which i think, even when extended fully, will only raise the lifting end of the arm by a foot at max.
Two, your jack is placed very low on the vertical support column, meaning, it's already more than 50% extended when the lifting arm is not even raised, that is, when it is parallel to the ground.
Three, you can increase the displacement of the moving arm greatly in proportion to the displacement of the jack piston by moving the jack piston closer to the fulcrum. But you will lose mechanical advantage in this configuration.
Fourth and last, the span of this hoist on the ground is very broad. It appears that if it were to be pushed under a vehicle, the front end of the hoist would come directly in line with the tyres of the car, preventing it to be pushed far enough to bring the lifting mechanism on top of the engine.
Jst a few observations, I hope you won't think I'm being too critical!!