I have found out, where you can get near unlimited lifes.
At the First Level Go too the 5th screen (take the Mushroom at the 2nd screen). Where the 2 Koopas came down, stand at the left side and look left (The shell most fly to the side, where you look, so its importent, that you look to the left side). Then when the first Koopa came down, jump that you land on it. Now the Shell will fly left against a wall and came back. Everytime you will again jump automatic on the shell again, so you get points. Then after some seconds you get lifes. After some time, the shelle will fly to the right, so it stop. But first time I have get near 55, second time after I have try it near 75 lifes. So you have many lifes, to make the next levels. Or you do this trick again and get more and more lifes.
Here 2 pictures, where it is. Just jump the first time on the shell and everything will go automatic.
Now have fun to finish this game. Maybe it work at others screens too, but I have found it out in this with luck. Just play it and wonder me that this happen.
Found too out, that the life score roll. After maybe FF (not sure where exact, but think it end maybe by FF) it start again with 1. But when you lose this life, you are Game Over. So stop it, when you have a high life number. When you see letters and no numbers in the life.
But its hard to tell, when you must end. So you must try a bit, till you get a great life counter.
Maybe there are others levels later, where this trick work again, when you lose enough lifes.