So, depending on the backing material, it's very possible that you could order a set sized for the 21SE and make them work on the 1801. I recently removed mine (aftermarket) and the backing material is a very flexible plastic. If they're like that, I'd imagine the one in the bow would likely bend around the radius of your bow as well, just extend further aft on each side due to the smaller size of the boat. It's very unlikely that they fabricate a rigid backing material to the exact radius of the bow. Same with the side ones; they'll probably just come further forward on each side.
If you can get measurements on them from a dealer, you could likely mock it up and see what would fit.
Another option is to look at the SeaDek, Aquatraction or similar foam options. They'd take up less space by sticking into the walkways, and would still provide some cushion.