I'm no expert either, but I've owned a number of British sports cars, have worked on many as a hobby, and currently own two Lotus Elans, a Triumph TR6, and a couple of Alfa Romeos for a different perspective.
For two years I hosted and produced a podcast I created called Vintage Racing Podcast, in which I interviewed race car drivers who were winners on the track in period and today, race engine builders, race car preparers, vintage car preparers, and many others. I'm currently building, with much assistance, a Cosworth BDA engine with my good friend who recently retired after 45 years in the business, about 30 of those as a pinnacle vintage race engine builder and race car prep shop owner.
This all has shown me that an engine as "crude", one might opine, as the BMC engines were so ubiquitous and have been so popular for many reasons, including ease of manufacture, low cost (minimal development over the decades), and size (not much you can stuff into a Mini unless you stretch the subframe and change the front bonnet bodywork, which is done here to accommodate Honda engines by the Ricky Racers). The intake ports were also siamesed, but as mondogenerator noted, this has been copied in many more current designs to good effect (e.g. Cosworth engine I'm building). The limitations of Siamesed exhaust ports are much more difficult to overcome.
Interviewing so many in the vintage racing hobby/business and attending many years of vintage races (even serving as tech inspector) has also shown how much subsequent development has occurred with the mechanicals of each motor and car; performance today typically exceeds that in period. There are also those who simply favor one design over another, even if their selected car or motor is not as competitive as another.
Don't know about a 1.8 liter in a Mini, but they were of course used in many street cars (MGB among them), as well as race cars. I doubt it would be possible to fit it into a Mini, particularly because there is no integral gearbox available for it I know of, which is one of the most profound features of the Mini. I have seen engines mounted in the rear of the car using transaxles, but that certainly wasn't done back in period on the race track. If someone managed it back in the day, I'd actually be intrigued how the implemented it. I do know of a Fiat 500 with a Lotus Twin Cam stuffed into the rear...
