I had the 29mm ones and it turned like a boat. According to Rovers North’s website, FTC5151 was expected to ship in 9 - 20 days. So for either side, I just got an M12 bolt, a nut and a jam nut and their combined length is almost exactly 23mm. Turns on a dime now.

For anyone who finds this thread later looking for advice, the bolt thread is M12 x 1.75. And drill + heat + an extractor bit will get that snapped off bolt right out. Ask me how I know.