I do not have an M1A, but this is the scabbard I use.
http://www.amazon.com/byNcStar-Tacti...s=atv+scabbard
It was cheaper when i bought it if you can believe it. I got it so if it got lost on the trail it wasnt a big deal, so far its held up fine.I leave it attached to the molle on my pack, but im sure you could rig it to your atv.