-- Last Updated: Jan-05-13 10:12 PM EST --
I suppose the next step might be to see if the folks at Thule will tell you what force is exerted by their cylinders. Odds of success that way are slim though, because they will know in an instant that you intend to buy something from the aftermarket so they aren't likely to tell you what you want to know. But who can blame you for looking elsewhere? Thule will most likely charge you as much as ten-times what equivalent cylinders would cost from an average hardware or automotive-parts dealer, and if that turns out to be the case, they don't deserve your business anyway.
Any chance you can find a fellow paddler with a Hullivator, so you can just remove a cylinder and test it on a scale?
For reasons I already described, my hunch is that you'll want cylinders with load rating toward the high end of what's available, if as you say, the maximum you can get is 130 pounds.
On the outside chance that you are really good with trigonometry, or if you have a friend who is, you can calculate the load rating of the original cylinders by measuring dimensions and angles of all parts of the device when the parallelogram is fully open, where maximum lifting force is provided. Assign a 20-pound force to the direction of lift when the device is in that fully-open configuration and work out the other forces from there.