Ahhhh takes me back - we used to use a b'stard file to shift the bulk of the cut then a sheet of fine (ish) emery cloth followed by coarse to Fine grinding paste on a sheet of glass
Once we'd done that we'd lap the barrel to the head with fine paste
Yes thats another way of doing it, lapping it to the barrels but using a sheet of glass with course wet & dry first. BUT you realy need to recut the chambers to the right volume 25cc in the case of the RD or it will be melted piston heaven.
But thats only one job I want to do myslef, got so many more projects id like to do.