That roller doesn't do very much for bale forming, as evidenced by the paint still being on it. Look at its position. By the time the start of the bale gets to it, it's barely helping to roll it at all. Maybe directing some of the loose strands. Then once it's formed properly, it's doing absolutely nothing.
Ergo, it isn't all that important to be replaced exactly life-for-like.
water has got into that roller somehow. Maybe through a pinhole, or something. But it happened because someone left it outside all its life uncovered, most likely.
"Oh but machine sheds are for rich folks"
Mhhm but everyone can afford tarpaulins...!
If it were me, I'd have that thing out of there asap, get some pipe close to that diameter, and make a new roller. Wouldn't cost a crazy amount. It doesn't have to be amazingly accurate - just has to have the ends machined true to one another.
You could locate some pipe, make the end spigots yourself (oversize on the shafts), weld up the spigots best you can, and send to a machine shop for finishing.
I wouldn't even bother with texture. Not this year anyway. I honestly can't see it making much difference.
EDIT:
in fact, you could do it another way. Make a solid shaft for power transfer, and attach the roller ends and pipe to it afterwards. It's not actually doing much "rolling" so you could make it a loose fit with grub screws, removing the welding distortion from the equation. That way, if you have any problems with bale forming, you can remove it and texture it later, or re-make it. Either way, you get out of the hole you're in.
I mean, you could even just make a plain shaft and cover up the hole with some sheet metal. Get away with it, and sell the thing before it breaks something else.
There's loads of ways to do it. Don't be boxed in by yourself Sam, that's what you usually do...
How did that disc mower work out by the way? Did you try it, or did you just sell it again?