RM125, hone cylinder before new top end???

Usually, you don't have to hone and sleave it. Running a brush through it works fine to get the glaze off. Yes you do need to get the glaze off but re-sleeving isn't necessary unless there is obvious damage to the cylinder head, scratches that are cut due to piston movement or breakage. That new of a cylinder head shouldn't be that worn out, but there is that chance it could be. Look at the top edge of the cylinder head and you can tell if it is wearing through the factory coating in the head. Re-sleeving has its advantages becasue if you ever have a problem in the head, you can have it re-sleeved and save the head, re-sleeving also allows to oversize your piston if you desire.