50:1 is correct for the Supa 5, but the older ones 33:1 is more the mark.
800 miles is not far. Might be wise to ease it in longer with a tad more oil for good measure.
I remember when I bought a Yam. DT175 in the seventies. Ran sweet from new, but pushed it a little after 5 or 600 miles and it partially seized. Pulled over and let it cool for ten minutes and it started up fine. Never had problems again. Maybe just need a little patience.