I wouldn't try to hide that hole with just paint. Start with a bit of fiberglass cloth and your favorite resin (I often use JB Weld), to put on the back. Then you can try to fill the hole with paint.
Or, slightly different approach, get one of those little Union Jack emblems used on later Triumphs and stick it over the hole.
Not perfect by any means, but better than the bare hole, IMO.