Hello, what would be an indicator that a top end rebuild is needed and is it more age or mileage linked. My bike runs ok but is so noisey it makes it not movie to ride for a long time. Thanks Tunny

Loud rattling from the cylinder area is the best indicator.  Apart from that, it is a case of situation and consequence.
Poor air cleaner maintenance, inlet leaks and a lack of engine warming before asking it to produce 50%  and upwards power will destroy a piston and cylinder very quickly.  Often in as little as 30 or 40 hours.

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Piston on the right from my own DT100 restoration is a direct consequence of all of the above problems.  Bike has just 2700 miles on the clock.

Great photo, ok I’m going to have a look at a rebuild. I think it will make a huge difference difference. Many thanks for the info

If you look in the owner's manual, the maintenance schedule generally says the engine and exhaust should be decarbonized after like the first 1000 miles and then every 2000 miles after that.  

This maintenance is rarely done.  

If you're pulling the head and exhaust to decarbonize, it's not a whole lot more work to pull the cylinder and have it and the piston assessed by experts like PJME in the UK.  

If piston/bore measured out of spec, then recondition.  Otherwise you can reassemble and ride it more.

One critical thing to check with cylinder off is the big end rod bearing.  If the engine ever sat with moisture in it, just a spec or two of rust on the rod bearing will cause it to fail very quickly and it could be the cause of your noise.