Man, finding legal spots for older or niche manga can be a real hassle. For 'Arata: The Legend', it's definitely tough. I'd check out the Viz Media app or website first—they have a huge Shonen Sunday back catalog, and I'm pretty sure that's where they released the official English version. I read a bunch of it there a few years back. Might require a subscription, but it's worth a look.
Alternatively, some digital libraries like Hoopla have Viz's manga catalog. My local library card got me access, and I was surprised at the range. It's a bit hit or miss depending on your region, though. If those don't pan out, second-hand digital copies from platforms like Amazon Kindle or Google Play Books are a solid last resort. You own it, it's legal, but you miss out on the subscription model's breadth.
I remember hunting for this title specifically and feeling that frustration when it wasn't on the big streaming services. Hopefully Viz keeps it in their rotation.