unovolo, I suspect it’s pretty much the same engine as the one they put into the show/program car. The standard Mexcio engine was pretty dull to be frank, around 88bhp IRC. With the modding and twin 40 carbs you can easily get that up to 140-150bhp perhaps slightly more. So, if you think of the difference almost double, that would produce the time gap between the two cars head to head.
I had various engines in mine, several cross flows and a rather expensive and disappointing BDA. The best was one I built using a Swaymar cylinder head, went exceptionally well much better than the BDA until a conrod broke and destroyed an expensive engine. I noticed on the rebuild, that the Harris’s used a steel conrod as opposed to the original cast iron – these cost about a grand a set alone!
I would hazard a guess that you could spend 4-5K on a standard engine to bring it up to the above spec. It’s so much cheaper to modify engines these days – a few hundred for re-mapping!