Yes they are replaceable! What size they are...I have no idea. IIRC not to long ago they switched to a larger bearing. Also the Tekno Neu's and CC/Neu's have bigger bearings
TeknoRC Neus have 5x13x4mm bearings front and back.
Castle Neu 1520 on 6S LiPo Powered Gmaxx (Nitro Killer)
Predator with OS .21TM
Supermaxx with Mach .26
Revo with OS .18TZ
Kyosho ST-RR Conversion
Ofna CR with Tekin ESC/Motor (2)
make sure they are flanged bearings which ever ones you have and if they are the 5x13x4 and you cant find any let me know, i have a tube of them, you can pm me.