8 cooling fins on the cylinder => it's a 250. 350s have 9 fins, 160s have 6. I'm not sure what 175s or 200s have but they're pretty rare in the US anyway. You can also see the boss for the breather tube right behind the alternator wire. 160s don't have that.I'd think it was a 160, maybe 1965 era. The cylinder looks small. Measure the bore. 160 was 63.5mm I think.