Typical design of a sickle mower is a pto drive to a small rotating flywheel with an off center attachment point connected to a "pittman arm". The other end of the pittman arm is connected to the sickle bar cutter bar with the triangular shaped cutter blades attached. Turning the pto causes the cutters to move back and forth. That design with a pittman arm will work at a little angle, but nowhere near vertical. There are some mowers with belt drives or hydraulic motor drives on the end of the cutter bar. Those will work at any angle.
If that does not make sense, turn the pto shaft by hand with the cutter bar down. Then raise it up. If you can still turn the pto shaft by hand with the cutter bar vertical, then you are probably good to go. If it cannot be turned when vertical, then it is not going to work for you. How big are those limbs you want to cut? A battery powered hedge trimmer might work better for you.
Be careful with your fingers when moving the cutter bar up and down. Mowers with a pittman arm allow the blades to drop by gravity when the cutter bar is raised to the vertical position.