I had exactly the same problem recently with a A12 rebuild I was doing of an A12 that had been sitting in the elements for a few months.

I just had to hone the cylinders and they came good again.

I took lots of pretty pictures and put them on a website

http://mildman.datsunprojects.org/

Check it the A12 strip and A12 reassembly bits.

My pistons were stuck in the bores solid and I managed to get them out, clean them, re-hone the bores the pistons moved nicely...although I never did a compression test the pistons seemed to be pushing the air nicely.