The model number would be a help. If it's an old green 10hp, then it would be 1949 or newer.
QD-10 1949
QD-11 1950
QD-12 1951
QD-13 1952
QD-14 1953
And so on. Check the QD number then subtract one.
red/maroon color definitely places it between '55-'58. They went to white, fiberglass cowls in '59 through the 60's. Here's a great link that can help you nail down the exact year and give it an all around tune-up.
http://www.leeroysramblings.com/johnson_QD.htm