What kjnd of bearing should I use?
A slim YYF or YYJ bearing. If you have a regular width c sized bearing in it, it will be very slippy, almost impossible to come back.
Goto the YYJ bearing page in the store and it says there
EDIT Size N
Size N is considered C Slim…
Any C slim will work.
What happened to your original? Because the original is perfect.
What everyone else said, just the YoYoJam C Slim one will do 
I see people say things like this a bunch and it irks me, not everyone has or wants the original, there are replacement parts for a reason, it could be dirty and they don’t want to clean it, could have lost it, could have broken, etc.
There are three different N-sized bearings(half-width C) bearings.
There is this 8 ball one that came with my YYJ DM2 nearly 2 years ago, which is either similar or identical to the ones that came in mine and my kid’s ONE and apears to be all steel. Then there are these 10-Ball ones that have a more surrounding steel cage system and these have come in many of my newer YYJ yoyos that use two bearings, as well as the Go Big. Lastly, there are these bearings with a plastic shield that have been in my Classics.
You want the all steel 10-ball ones. Clean it out super good. I’ve only had 1 not pass my minimum requirements of 20 seconds of spin on a flick.