I've replaced the bearing in mine.  I just removed the tensioner, unbolted
the pulley, removed the circlip and the bearing came out easy enough.

I didn't write down what it was though :-(  I just took the old one to my
local bearing supplier and he matched it up, cost something like a fiver.

I may still have the box in the shed but it'll be quicker to remove yours and
look as I'm not at home.