Probably the cheapest option but I've no idea what the quality of them is like.

If the speedo works I doubt it'll be the cable, sounds like the speedo unit has suffered some damage, probably easiest to replace speedo unit unless you can find another spares one to cannibalize for parts.

I may have one, stripped all the starter motor parts off my engine. Can have a look if you want?

More mechanical advantage from a longer clutch activator arm maybe? Smoother cable routing?

I believe there expensive due to being a oddball size, something .5mm wide I think.

Tyre size? My speedo under reads 3mph of actual speed, due to different size tyre, if you factor in most speedos over read. Could be 6mph+ difference.

Sounds like it's running lean at some point with choke improving things. You need to work out what throttle position the bog occurs and try richening up the part of the carb that controls that throttle position.

I'd check you've got a healthy spark kicking it over with kill switch set to run. Does sound electrical with it being connected with touching switch.

I'd say the pressure noise would be the plug igniting fuel in cylinder, noticed on my dtr it would produce a weak spark couple times after kill switch was set to off.

Is the clutch hub and basket lined up correctly seem to remember you can fit the dt one 4 different ways but will only work in one position