I had the same problem on a TC 1558cc engine.
I had to take the pistons out and very carefully ease the piston rings out of the grooves in the piston, and clean up the rings on emery paper and the pistons using wire wool.
I know taking the pistons out is a big deal, but as the space between the pistons and the bore is very tight, any liquid you pour into the head will find it very hard to ease or dissolve the surface rust.
Unless you can get rid of all the rust, the rings will not move independent of the pistons, so will not seal correctly. All the rings have to be released, or the piston will not give a gas tight seal, so the compression will be low.