Unless the C band dish you need to rework is totally trashed it will out perform a 4 footer in both gain and selectivity hands down. The mesh will have to be pretty messed up and in a lot of places to be noticable to a C band signal, and it will still have a tighter beamwidth regardless. Another plus is that it is (or should still be if it has been undisturbed) properly aligned to track the arc.
One less thing to have to do.................lol
Cut the vines and let them die before you attempt to pull them out. They will decay and break loose easier with less potential damage to the mesh.
I've seen some pretty cruddy looking jacks still run after sitting idle for years. Dont count it out until you've tested it. If you dont have an old analog IRD or some other conventional antenna moving source, a 12v auto, lawn tractor, etc battery will move a working jack, all beit slowly.
At the risk of stating the obvious, reversing the polarity will reverse the direction of movement. If you can't get a response, check the mechanical limit switch for proper operation.
Be sure to use the "heavier" gauge wires (appx #16) for powering the motor. The smaller ones (appx#22) are for the reed switch (pulse) and applying the driving voltage across the switch will destroy it. Even if the jack is "frozen" you still may be able to free it up with some effort.
Once you can determine that the ol' gal is service worthy you can invest in repair/replacement parts as necessary.
Good luck, keep us posted.