it depends , i think if i had a bike that old in that good shape, if i could afford the cost, i would go with one of the good rack mounts, with a belt drive.you need a front brake, which requires a bit of welding to install on your bike. if you do go with a frame mount, i would suggest a mount along the lines of this one (first picture).
http://zbox.com.au/innovations.htm
as to not damage the frame.
your left pedal crank arm will most likely hit the side of the happy time engine, but check for sure by checking the distance from the center your seat post tube, to the inside edge of you pedal crank arms, you need 3-5/8 inches to clear the sides of the engine, the left will be the one, if you have that problem. to be honest, i would park that bike, & invest in a newer steel frame bike, with front & rear hand brakes, not coaster type, (you have to enlarge the sprocket hole to clear the coaster brake arm.) & save that bike , you don't find em every day. if you could just keep that bike like it is , till you are old, you would be happier. check your local laws regarding motorized bicycles for lots of states are different