A £13/£16/$35 (I found different prices on line) Brightspark requires the armature to be removed when the first one is installed in order to snip the lead to the existing capacitor. After that, if the magneto is re-magnetized, factory-fresh functioning will be restored, at least until/if that capacitor fails. The advantage a Brightspark has if/when it does fail is it can be replaced on the side of the road with another of the £13/£16/$35 capacitors.
To install an internal capacitor, as was used by the factory, also requires the armature to be removed, but factory-fresh functioning also is restored if it is then re-magnetized. The disadvantage of an internal capacitor is it cannot be replaced on the side of the road. However, capacitors of the type on which I conducted extensive accelerated stress tests that showed they would last at least 50 years and 150,000 miles in actual use, at a cost of $2.70 for the necessary pair.
Something that costs £26/£32/$70 for it and a spare that can be replaced on the side of the road if/when it fails. Or something that costs $2.70 and that doesn't fail. The choice is yours.