Hello Brendan and Dave

Any automotive engineering shop can do this work.
More experienced lawnmower repairers can do it too.

The issue is securing the block for an accurate drill and then screwing in
an insert. Prices can vary. At least get some quotes.

In term of originality, it won't be the same will it?
However, a 'pot' muffler will not interfere with restoring the specification at
some future point. It's up to you and economics.

[Linked Image]

Hope this helps.
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JACK