I lifted this quote from Simon Hadfield replying to the question. "people forget that in period John Miles in his 26R beat Jack Sears (who had the fastest Cobra in Europe at that time) at Brands Hatch and that included a pit stop (!), and at Villa Real dusted the local hot shot in a Ferrari 250 LM. So the impressive part is not the speed but the reliability And a 26R has a 4 speed gearbox......and as an Elite is as fast as a (legal) Austin A55 sports then surely the Elan should be at the very least 10 seconds a lap faster?"
Being a multiple winner in many and varied historic cars, it would be foolish to ignore Simon's views.
For my part I am somewhat bored with the whole issue of development versus cheating, in historic racing. We know that many people run larger engines in their cars for various events and if the organisers are unwilling to weed them out, then either us competitors need to protest or shut up.
Borrowing a post from another poster (GrantP) who said more generally: "The more one thinks about it the more one surely comes to the conclusion that few non-club racer owned cars out there in Times Historique stayed the same for more than a meeting or two at a time. Maybe only a race or two." In essence he's asking "what is originality"?
So, and realising this isn't just about engines, but, if someone makes the pistons in their car out of "unobtainium", surely that is development? If however, they make them bigger than they should be, then that is cheating.