It might be worth noting that the o-rings used in Japanese carb are typically made from fluoroelastomers with Viton being a common one in my experience.
You're a chemistry guy so I doubt I'm telling you anything you don't know, but for anyone else reading this, the above is important because Viton is well known for its heat resistance and stability when subjected to aggressive fuels and chemicals. Generic o-rings made with natural rubber or other less suitable compounds will tend to break down and leak when subjected to the heat of engines and the solvent properties of the hydrocarbons, ethers, and alcohols in modern fuels.
Good carb kits and the o-rings in them are usually worth the money even it the cost seems high.
Just thought I should pass that along.