There should be a release screw/valve for the hydraulic system. Usually it's on the unit/pump. You will need to open this valve and then manually lift the motor and tie it up with some rope. If you get it up pretty high you may be able to close the valve and the lower unit may stay up if there is enough pressure left in the system. I am not an expert in this but I have had to lower a unit when a fuse popped in the trim/tilt up position. The only way to get it down was to open that valve.
Yea I think you can do it either way , loosen the valve and jack up the motor then tighten the valve back. It should stay up on its own.
But release the Jack slowly just in case.