As Jim said, the simplest thing is to get a new ring, either from MECoA or Frank Bowman. That way you will have two engines in running condition.

If you insist on making one from the two you have, the best way to move the good ring into the newer engine is to slide the liner and piston out of the older engine as a unit - do not pull the piston out - and slip the whole thing into the newer engine. You may not get the sleeve exactly aligned, but by leaving the piston in the ring will turn with the cylinder and remain in the same relative position, you will still have a piston ring that has been broken in. If you pull it out, either to put it in the liner that is in the new engine or to make it easier to change the sleeves, it will never be in the same position, it will have to wear to a new seal in either liner.

While you're at it, don't just get one ring, get two and a new crank case. Put the new ring in the newer engine, and replace the crank case of the older one. That way you'll be able to use the bolt-on muffler on both of them. Yes, all the 0.40 crank cases will interchange, cost is about $25 from MECoA.

Bill.