I was away to attend a large fair for one week and in the meanwhile you have done good work 😉
Are the lines sprayed, pinstriped or decals?
Regarding the main jet, I can only provide data for the BS34 carbs which are fairly different from the BS38 so that wouldn’t help.
As you stated the standard main jet is 125. If you add 5% on that it will be 131,25. I would go for testing a 132 when using a stock exhaust. I you are using a “high flow” undamped exhaust it may be larger. If you have a 128 on hand you can try it as well.
You can only find the correct size of main jets when driving full throttle. As long as you just test it when slowly driving round the block you will not find the difference.
With K+N filters attached and a to small main jet you may help yourself using some tape round the filters to reduce the amount of air they can soak in. You may cover half of the filter to feel the difference.