Chain Tensioning Blues

1980 AMF Roadmaster moped with 66cc engine kit

It's really hard to match tension on two different chains (one on each side)

My pedal chain (from 1980) finally broke yesterday. I bought a new chain and sized it as close as possible to the original chain. I even bought removable half-links (for both drive chains) in a vain attempt to get that perfect length and tension on my new pedal chain to match the tension of the motor chain so I could align the back tire perfectly in the middle and not have the tire rub on a chain from either side. I can get one side tight by removing the links and half-links of one, but then the other is way too loose and vice versa. The proper length of adjustment needed would be exactly the size of one quarter-link, but that is impossible to find... 0.25" inches doesn't seem like a lot but it's the difference between having the tire misaligned and rubbing on the chain - or not.

Does anyone know where to buy a quality replacement drive chain for the chinese motor that is better quality and doesn't stretch as much as the old one? Also I believe the stock motor chain is a #41 (1/2" x 1/4") and its rollers seem a little too beefy and wide for the sprocket teeth it mates with on the engine. Any help/suggestions?

I'm hoping the new motor chain will be able to make up that 0.25" disparity in length between chain A (old/stretched) & chain B (new)

I was able to match up the tension on both chains by replacing them both at the same time. The fact that one new chain had been throwing off the old chain was just a difference in the slight amount of stretch the old one had when compared to a new one of the same kind #41 chain. I was also able to lengthen my wheelbase (as far back as the notches in my frame would allow - about another inch. This has given me better stability by adding another link to both (new) chains compared to the old ones. Now it is a little easier to maneuver and stability and turning have also increased.

I like that derailleur idea - I may have to try that sometime.