Cant you just remake the recirculation tube? as far as I know it only feeds the balls back to the start, any preload is normally provided by the ball size, I would have thougth a small length of brass or copper pipe that you can get from model shops would be at least worth a go before you shell out all that money :o)

I have a similar looking one at at home but I'm not sure its 2.5mm pitch. I'll have a look when I get home.