Back in my days of erector engineer (always as the contractor, never as the client), I've done that work at the clients' satisfaction (very stringent clients, by the way) by means of a cylindrical "mounted stone", i.e., a small cylindrical grinding stone installed on the tip of a steel rod. The mounted stone, in turn, is installed on a pneumatic or electrical portable grinder.
The grain size (or "grit", as they say in the USA) I don't remember now, you can discuss it with a reputable mounted stone distributor in your area.
Before starting, expain your client what you intend to do.
Do the job with care, so as to obtain a good surface finishing, which of course will never be as a machined one. Never mind, orifice meters are flow metering devices that work with an error margin of 1-2%, and sometimes even more, depending on the working conditions. So, the finishing obtained with the mounted stone is enough.
Giovanni S. Crisi
Sao Paulo - Brazil