Hello & Welcome.
My boring man
will be not much use to you, as you are at the other end of the A1, however I ended up using Mallory Pistons, which after a lot of googling, appear to be equivalent to the latest level of the pistons for that machine, but at almost the cost of one Merc piston for the set of three!
Rest of the story is in Chewy's link.
Out of curiosity, which one let go?