I say, if your magnum opus doesn't feel like it's within your power to create, mutate it and modify it until it is!
I mean first of all, you can always grow as an artist (even if you don't think you can) and if the story is really that important to you, you'll make the effort. Train for the tougher things you know you'll have to draw later on.
And in the meantime, just simplify things as much as you can so you can get started. I remember once I wanted to do a fantasy comic set in some dark, cursed world, but I didn't feel up to drawing big furry monsters or detailed forests...so I made the monsters these vague shadow creatures, and filled the forest with lots of twisty vine things so I'd have something to put in the panel when I didn't know what to draw.
I've since dropped that comic, but even with my current comics I do similar stuff. I didn't feel I could manage designing an antique-looking house for DotPQ, so I made all the architecture modern and minimalist (technically, the story takes place in the present day, so it works).
And for my own magnum opus, Memor1n0, I designed an art style that sort of floats between cartoon and anime, so I'm always free to dial the detail level up or down depending on what I have to draw.
Above all, just remember that your magnum opus is just another comic to everyone else. If it turns out to be a huge success and gets famous and everything, you can always polish up the art later.
But if it takes the route of 90% of webcomics and just gains a moderate following, I think you'll be glad you got your story told, rather than worrying about making it perfect.