As a stop-gap – to at least keep you on the road – you could use a “four cylinder” cap which is more easily obtainable.
(You could then run a couple of “dummy” plug leads under the engine – that WOULD get every one guessing)!!!
Historical footnote:
It was way back in the early sixties that I obtained from Joseph Lucas in Birmingham a quantity of distributor caps for The club which we sold for £1 each – so starting the spares scheme.
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