Find a replacement gas spring/strut. Just measure the stroke of the one you have and find a match. To figure out how much lifting strength you need, place a rod/firm stick at the point where the strut will be mounted, with a scale under it. The amount of weight pushing down, at the mounting point
, is how much lifting strength you need. Don't just measure at the far edge, as the leverage/strength needed to lift increases as you move toward the hinge point.
Here are some examples;