WARNING: This will not work with the Z height detection, it is too long for that. Will make a new version, one day; but as I don't use the Z height thing, it won't be today..
One of the idler on my printers X-axis broke, from the internet I see this has happened to others and my experience with support is "fix it yourself" ... so here is the part:
Need to have the pillar holding the old bearing break off completely, or help it dissapear. Add some long-ish M3 screws and locking nuts - and print one before you need it.
Put nuts in, put belt in, slide the bearing and gears through the 10mm hole on the thin side and seat it against the 9mm hole on the thick side. Thick side up to prevent bearing escaping. Bearing should portrude a little on the bottom and ride in the groove.
No idea how long it'll last, printed it in ABS and made two spare..
20190401 Added an alternative design to recover more travel and try to constrain the two bearings better, the initial damage caused one of the bearings to slip off the idler shaft with the previous design.
20190612: Possible alternative solution: https://www.thingiverse.com/thing:3987766