have heard you can go to 79mm bore, thats 6mm oversize, but you need a custom head gasket for this. 78mm is largest for standard head gasket. I guess it depends on your particular block but this should be quite achievable. I know JTS has an A12 block over 1600cc reliably!

We used to get 4mm overbore, to get 1300cc.
I found that about 1 in 5 blocks would take it, you just need to line them all up and get them sonic tested (which aint that expensive)

The cores shift during casting, which leads to them being thinner on one side of the bore...you need to find one that hasn't shifted much....

3-4mm overbore can be done but check your block out at your local machine shop before going 4mm as some castings may be too thin in some areas to go that far.

Have an a12 block sitting in my shed just wondering how far you can bore them or the best way to go about it cheers