Northeast Vintage Cycle ebay store has reproduction from Japan piston and ring sets for the 1000 in std. or .25 over or.75 over or 1. over, set of four for 350. dollars.
Everything and more ebay store has used gl1000 parts, like a complete rotating assemble, crank, rods and pistons for 48. dollars. They list a lot of gl1000 used parts.
Depending on the miles on your engines and how well they were cared for, personaly I would see if the compression is being lost in the heads before jumping on an overhaul as the bottom end of the flat four honda engines are tuff as boot leather and hold up for many miles of use. Hope this info helps. Good luck.