Hey Lou, Welcome
You can find the weight of various outboards here
http://www.nadaguides.com/default.as...r>c=ob&any=0
4-strokes tend to be heavier, but the Tohatsu/Nissan/Merc engines seem to be very light. The 4,5 and 6hp all weigh about 55+ lbs. That's probably about the same as your Johnson 6hp 2-stroke.
The 2.5 and 3.5hp Tohatsus weigh about 41+ lbs. Not a huge weight savings (although signifigant if you're trying to manhandle the thing on a pitching boat).
http://www.tohatsu.com/outboards/index.html
To get an engine around 30+ lbs you would probably need to find an older, small 2-stroke like Craig has