Any machine shop specializing in 'engine' work should be more than capable of doing this sort of job. Bring the pistons in with you and be specific about the clearances you are after....the only thing to watch out for is that sometimes they see motorcycles stuff as a training ground for the 'new guy', it usually isn't a problem, but if you are worried about this, give them a line about the jugs being rare and expensive to replace and that you want their 'top machinist' on this job.