According to a thread called "Murray serial number project" on thecabe.com serial number "MO6 140 60056" indicates 1966.
In 65 they were changing there serial number style and by then they were on the left rear dropout. Some 65 bikes had an MO5 which stood for Murray of Ohio 1965. After these 3 digits was a 2 or 3 digit model number followed by and X and a 1 or 2 digit number that I will discuss momentarily.
MO5 = 65
MO6 = 66
MO7 = 67
MO8 = 68
In 69, they changed the layout yet again. They dropped the O. They also went to a 4 digit model number.
M9 = 69
M0 = 70
and so on.
So, in your case the format should be
MO6 = 1966
140 = model number
60056 = ?
Finding a 1966 Murray catalog to verify that such a bike was made took me to "1966 Murray Specials Catalog" at tapatalk.com
Sadly, the page is cut off so that the model name is not visible. Even cut off the bike's name looks like "MONA ---" rather than "MONT ---"
What we can see of the description indicates that there was a three speed made in 1966 but the men's model was metallic green - not chrome.
Squinting at the head badge in the picture just right might convince you that the head badge is a match.
There is something that might be a model number in the picture "6-130X66 BOYS 26". If that's the model number it might be that the "140" on your bike refers to a chrome three speed Monterey and the "130" refers to a painted some other name.
Lots of speculation there.
Murray made different bikes for different retail outlets at the same time. The same bike might have had different model names or finishes.
On a side note,
These bikes did not have a good reputation for standing up under rough usage.
Motorization will put stress on this frame that it was never designed to handle. It would be wise to select a different frame to motorize.