Teledyne iron nitrides their barrels, as they are unlined. That is another option instead of chrome plating a cast sleeve for durability. It hones very nicely without special tooling. It is what Ariel Corp does to their gas compressor cylinders and they manage to get .040” to .045” hardness depth with the way they do it. I don’t remember if we checked hardness depth at Teledyne and or what depth we were achieving.On that sleeve on the 125 Mac I used grade 50 fine gray cast iron which is exactly what Harley D used on iron cylinders . I have no idea what for example McCulloch used . This has been kicked around quite a bit lately .It might be LA sleeve could have something close .
I also have no idea what Honda or Ford uses for die cast in place aluminum cylinder blocks .
Another option could be 4140 steel which is used on aircraft cylinders but it's chore to machine and finish .