A silver cap with gold trim on a black barrel is a factory specification and not something put together after the event: the pen was sold as the Traveller Doué. Doué and Solitaire caps of this kind cost more than the black caps with gold rings, which makes swapping one for an ordinary cap a poor bargain. The 146 and the 147 are close relatives and differ chiefly in their filling system, the one a piston filler, the other a cartridge pen.
Montblanc does not sell caps separately. A replacement is supplied only against the defective cap being sent in. The reason is that counterfeiters would otherwise get hold of genuine spare parts and could fit fakes with original components. What can be done is to exchange an existing Solitaire or Doué cap for a standard one, though there is a charge for it.
Buying a cap second-hand calls for caution, because the threads are not the same across all years of production. Collectors report that the 146s of the 1980s had a different cap thread, so that a damaged cap meant the body being changed along with it; the second-hand market accordingly throws up examples with an old nib but a later cap and a later body. Caps of more recent production ought to fit the 147, but there is no guarantee of it, so it is worth trying one on before buying.