Calibration of maps

I am trying to calibrate the map at
http://www.maps-continents.com/america-political.htm for use in Ozi
Explorer, but I am having some problems, presumably because the map
projection used is based on a globe and not a flat page, where 90W/0N, 0W/0N
and 90E/0N are not all on the same straight line but otherwise on a curve,
although some pre calibrated maps I have work fine and have a similar
projection (Bonne?)

I need to know the projection (Mercator?) and datum (currently left as
default WGS84) - which aren't listed on the webpage, anyone hazard a guess?
Failing that, any pointers would be great.