The badge screws are 3BA. Jerry Mortimore used to do, and may still do, screws with a head profile similar to that of the original solder blob. I would advise use of these if you can get them. An alternative is to use dummy screws and stick the badges on with silicone or similar.
The original way to hold the tank badges on involves making some brass 3BA screws. Use a suitable drill or countersink bit to remove the plating around the screw holes of your new badges (having bent them to the shape of your tank, hopefully without cracking the enamel) and screw lightly to the tank. Apply flux to screw and badge. Heat just enough to raise the temperature of screw and badge to take solder and apply to the joint, removing the soldering iron with a flourish so as to produce a nice dome over the screw of solder. I found the only way to do this was to use electrical multi core solder using an old fashioned copper soldering iron heated flipping hot in the domestic barbecue. It can be done, but it is far from easy.
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.