It looks like I've done a Big end on my CA18, so i'm toying with the idea (along with balancing etc.) of doing an offset grind on the crank to increase displacement..........

Has anyone here done this - do you know if there is a standard piston / conrod available that will slot straight in to save machining the piston?

Do you have a particular rod in mind to use on the smaller diameter journal??

No, I have no info at all, I was just hoping that it was a common tweak & there was a standard rod / piston combo available. I have heard of people using L20 cranks & rods, then machining the pistons, but I was hoping for a cheaper option.

Big end on CA rods are 48m diam and L series are 53.1 mm. This means you have to have a rod with a diameter of 48mm minus the offset diameter reduction.
The only ones I can find that fit the bill are QR20 rods 45 ID and 152.9mm or QR25 rods @ 143.05 long with same diameter. The only problem with these rods is they are 22.7 wide vs 21.75 so they would have to be thinned down. Gudgeon diameter would also need to be checked.