There are, unfortunately, a number of "experts" on this here....
You can replace just the sliding portion so long as you use the same canopy supplier (either Van's or Todd's) that you have on there now. The shapes are different enough that they don't match well otherwise.
Level of effort? Drill out all rivets holding on the skirt and canopy. Drop the old canopy inside the new one (upside down) and trace the old shape to get a first cut. Cut, sand, finish until it fits. Reinstall - but prepare for your skirt not to fit exactly right, so get out fiberglas and re-do it. Repaint - your done.
It took me a couple of evenings to do all but the paint - I don't paint, so that was another few days at the paint shop.
