There really shouldn't be oil dripping from the cable, oil can wind it's way up the cable and into the speedo head and stop it working. It would seem that the bush in the speedo drive is worn and allowing gearbox oil to leak out onto the cable.

It sounds as if the rubber of the chain tensioner has broken away. You then get metal to metal contact which will not go away with the clutch disengaged.

I think the return pipe comes up inside the filter. A spacer in the bottom would need to be doughnut shaped, that is why I suggested a spacer on the top.

The top of the filter is 1 inch down from the top of the oil tank neck. The spring load button thing is 1 1/2 inch long and when on the filter the top of the button is 3/8 inch below top of neck. The button thing does come off easily but it come close to the bottom of the neck so cannot fall into the tank. I hope this is of some help.

I'll go into the garage tomorrow and measure it for you.

I can't for bikes but years ago my father in laws Morris Traveller threw a rod and took a chunk out of the skirt on number three cylinder. I just put a chamfer around the broken area. It run for year like it. Also when I was involved with a RT bus the same thing happened on the engine. A good chamfer enabled it to run for a few more years until we sourced another engine.

I believe that the A65 around 64 -65 had bigger valves for one year, it seems that it wasn't very successful and they reverted back to the previous size. I'd will have to try and find where I saw the information.