It all depends on how far you want to travel

StephenRC45 is in Plymouth

www.totaltraction.co.uk is in Essex. I'm happy with the rebuild Mick did on my MC18. He's worked on countless 2 strokes and 4 strokes

If it's just a top end you could do it yourself and send the pistons & barrels off to Poeten Aptec http://www.poetonaptec.co.uk/cylinder-repair.htm. Then ride it down to Stephen for a tune up
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Why do things simply when you can complicate them