Hello Fabien,
Thanks for your informative reply. Yes, in fact the tractor is owned by my uncle who is a competent mechanic/machinist so he could machine this, but asked for my help to see if I could find one to buy, to save him some effort.
I wanted to ask was the cylinder liner simply the 125mm version drilled out for a 130mm bore? Or was the engine block also different, so simply getting a 125mm version and machining to 130mm wouldn't work?
Anyway, thank again for your helpful reply, at least I know it's not likely to find one, so we can look at the other options.