Seized Pistons

Sparkplug

I strongly recommend boxing the pieces and having a reputible builder do the job. Mick Hemmings is a good choice as well as SRM but with the current exchange rates you may be better off with Colorado Noton Works here in the US. There is a lot of machining and fitting required on these motors and unless you've done it before it can be a bit daunting. The cost savings will seem insignificant (and it will be a lot less than you think anyway) when your Norton sits for months waiting for the rebuild to be over. Another added benefit is the peace of mind in knowing everything was done right.
Good Luck and Thumbs Up (Bultaco Logo)

Scooter

Have you got into my garage during the night and seen the bike or what?

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Didn't have to! I keep a couple souvenirs hanging in the shop from motors I rebuilt after the DPO smeared RTV all over!

Scooter62 makes a good point for a professionable rebuild, but since you live in Spain, I can see why you might hesitate to ship the engine to England or the U.S. to rebuild. A competent machine shop can handle crank and headwork, and the factory service manual is pretty complete, so if you have the tools and take your time and ask questions, you can do it.