Diamond Brand built the first Boy Scout Yucca packs way back in 1931 and they remained in use for decades, both durable and versatile, especially with the addition of an external frame. Many scouts built frames themselves using everything from metal tubing to scrap wood and occasionally even saplings or tree branches. When packed lightly the Yucca is more than adequate for as much as a week on the trail depending on how much food you pack in, with a frame they are good for most any length expedition if you accept them as they are and don't expect them to perform in the same ways a modern rig built from man made materials will. If you follow the same simple rules scouts of generations past did, a Yucca pack will serve you well and outlast many of their modern counterparts. Personal experience with canvas packs and sea bags leaves me noting that I have used the same canvas duffels for years while theoretically better bags have succumbed to the scrap heap due to everything from broken zippers to torn cloth that wouldn't take a patch. Unless canvas molds and rots, it can be repaired. The Yucca in particular is pretty accepting of most repairs from replacing grommets to sewing torn fabric and seems to riveting on new leather parts.
Canvas isn't hard to care for but it does have a few special needs. It isn't waterproof though it will shed water for a bit and will do a pretty decent job in the rain if you Scotch Guard it. I have also used either a rain poncho big enough to cover my pack or covered it with a black lawn and leaf bag. Either method works fairly well. All in all, water is not a friend of canvas. So, rule 1 is keep it dry. That means let it hang and air out at night, don't set it on the ground and if it gets rained on, hang it up and let it dry. There is usually a handy tree branch to hang things on. If not you can use a tent pole or run a line and hang it from that.
This leads to rule 2, keep it clean. repeating the mention above, don't set your canvas pack on the ground. It'll get dirty. Dirt can get in between fibers act as an abrasive and cut them leading to holes. Dirt also often contains components other than "soil", food and other sticky things often attract bugs and other animals that will damage the canvas. So, avoid that as well and if your bag gets dirty, wash it and hang it up to dry. It's canvas after all, not hard to clean. I've washed Yuccas by hand in the past and more than once successfully in a machine using a mesh delicates bag followed by line drying the pack. Just do your best to look after a Yucca and know they are not hard to clean if they do get dirty.
In spite of these rules, I have also seen a heck of a lot of dirty beat up packs that have gotten rain soaked, spent time on the the ground and been generally abused but keep on going just fine and still provide years of service before being retired to end up as "art" on a wall or hall tree.
As of this writing I was finding serviceable examples, some with frames, on eBay for as little as $12 and $15 plus shipping. I have seen a few of the 1307(D), 573 haversacks as well but the 574 is more common by far.