How about some good pics of the mower, and some explanation of what you want to raise?
A sickle bar usually needs to raise in two places, one at the 'heel' and the other at the tip. If the heel can be raised a few inches, like 6-8", then cutting can be done at a height above the ground level. This may now be controlled by just a lever, and this is what you want to replace with hydraulics.
If you want the end of the blade raised from cutting height to full height, are you interested in the tip reaching down an embankment (like the recent request for a mower to do the pond bank) so how low do you want it to go? And how high? all the way vertical for a travelling position?
Lots of things will need to enter into your design.
A pic or three or four will help. /forums/images/graemlins/wink.gif