I machined it from a new piece of round stock. Boring it out was quite difficult on a small ML7 lathe, very slow and time consuming. The internal bore isn't as smooth as I'd like, but as no one will ever see it, its not an issue. The bearing seats are good though.
One thing I did find out, I have two sets of the same bearings from the same supplier, however one outer race is slightly smaller, which means that it in future if bearing replacements are needed, bearing fit adhesive might need to be applied as I machined the bearing seats to the larger size. Not an issue for me, but does highlight the difference in tolerances even on 'precision' items.