John,
You might consider machining off a significant slice from the port face of
the castings and replacing this with a similar thickness of brass sheet.
This way, you can mark out and make the ports in the sheet, a great deal
easier than a casting. I would recommend sweating this on with soft solder
or using gasket paper. A couple of my engines have this, and it seems an
easy way to get it right when someone else has already got it badly wrong.
Getting the cylinder up to red heat for brazing seems overkill, and would
soften the metal and need a lot of heat.

Simon.

John Baguley <baggo@...> Wrote:
> The ports are going to be the biggest problem. I think the easiest way
> (apart from buying a new set of castings!) will be to machine a recess
> in the port face removing the old ports and then braze a piece of
> bronze into the recess. The ports can then be re-machined as normal.