Thanks for the info.
Yeah, if I could I would get someone to strip the engine down and replace the shaft but I dont have any money to do so :/ I'm expecting it will be the best part of £1000 as I cant do it myself.
I have no idea how well this weld was done on my bike but I dont see any leaks or noises at the moment and the gearbox changes gear quite smooth and slick although 1st and 2nd have a bit of a clunk.
The engine is silent and not smoking, so I am hoping I can get 20,000 miles from this current sprocket problem free, which will be 3 years or more and then I can consider a rebuild.
I have only done 1000 miles so its still early days...fingers crossed and maybe at some time I'll have a look for some replacement parts.
I'll update once 5000 miles are done and hopefully all will be good
Keep your eye"s peeled for a 2nd hand complete engine that's being sold off because of rotting wheel rim"s etc. It will be easier than a rebuild.