No Hammer.
Hey Felipe, Big Red again.
I don't know FOR SURE if you ruined your bottom end or not. I've never hit a crank end with a hammer. If you knocked your crank off to one side, I think it would make the piston rod in turn push the piston hard against one side of the cylinder wall. I think the easiest way to find out is to take the head off and inspect the cylinder wall and piston clearance. If the piston it not centered in the cylinder then you have a problem. If so, and you want to chance it, "AS A LAST RESORT" hit it on the other side to knock it back. (use a rubber mallet.) I think otherwise, Rebuild or replace.
But BEFORE you do any of that, Make sure there's nothing else wrong. Make sure your carborator is not clogged up with stuff. Check for air leak's along your intake. Be sure your getting a strong spark, Those cdi units are good for going bad. Replace the spark plug that came with the engine, They are junk. After you do all that, Then inspect your cylinder and piston.
I do this all day long on my workbench, I've just never tried to "tell" someone how to do it in a forum. And please, NO MORE HAMMERS.
If you wind up replacing it there are a lot of good suppliers of engine parts. If you want the cheap $50. bottom end just go to e-bay and type in BICYCLE ENGINE. Thats what I do becouse it's cheap and easy. I think BOYGOFAST has the $50. bottom end and they ship fast. (I think shipping was $20. on that.)
Anyway, GOOD LUCK. I hope things work out for you.
Big Red.