Have you checked with motorcycle dealers? I work at one in Vermont and we have a guy that does all our two stroke bores. He is set up to do bores close to 4" in diameter. He is on vacation this week but I can ask him when he returns.
I strongly suggest that you pick somewhere that has experience with boring outboards. Otherwise your mileage may vary! Call a locale Merc dealer and see where they get their machine work done.
try Rons marine in portsmouth VA. he has bored my stuff since the late 80's and for years was an autherized OMC boring and resleeving center.
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