Ah, one of the rare gearboxes with a genuine Goldstar kickstar quadrant. The same box should also have ported and polished screws with twin overhead clutch levers. This particular box was probably the one fitted to the very bike that broke the world speed record in 1950 with both tyres flat...................get the idea !!!!!!
You can get a gearbox out of a plunger without taking the engine out but its easier to take the engine out as you have to remove all the bolts anyway. You can remove the internals of the gearbox with it all in the bike, depends on what you need to do to the box.