Actually I tend to not believe brandon...
I've had 2 old Minis, raced 1 in a local rallye event and tuned them both very slightly (998 engines, first had 48hp, the rally one slightly over 50).
First of all I prefer the old Mini to the bmw 0-series because the Mini was revolutionary. The concept was to fit 4 people and some handbags into a car shorter than 10 feet. So they had to put the engine in sideways and mounted it directly onto the almost-syncro-4speed gearbox. The springs were made from rubber, the interior was minimalistic and cheap. That was all of the 600kg Mini. Todays 0-series is as big as all the other "compact" cars, weighing almost 1.3 tons, has been built without ANY revolutionary thing to it (it hasn't even got an Aluminium engine block...), just the design should make people thinking of the olden days.
Now I've had 2 of the proper Minis. And I was thinking of getting a new engine in as well. There are quite a few people who actually CAN do it, but there are 10000s of people who claim to have built one themselves, no sign of proof though (sorry brandon, nothing personal. You're not the first). Fitting a camshaft, mounting some proper pistons, getting the twincarb to work well (I had a HS4, wanted a double HS6 :-) kids dreams), these are hard to do things on an engine. But ripping out half of the front subframe, putting the engine in and fitting the new welded subframe to it, making a whole new wire harness (sp?), this is really hard work, impossible to do properly for amateurs like most of us. Don't forget you need a proper racing clutch which is light enough to fit the bike engine and sturdy enough to pull the car... You might need a reverse gear. For this some funny guys put a Mercedes starter motor into the car, so technically it's a hybrid :-)
The last thing I've seen was "...in a car that weighs less than half a tonne". No mate, this is not possible. I've ripped out the roof lining thingie, carpets, rear seats, mounted windows from macrolon (plastic sheet) and the car was still at 570, exactly back to 600 after mounting the rollcage (640 standard). Flipfront is 15kgs lighter than the stock one. I know much about tuning that thing, and I know why many people stick to rallying Opel Kadetts, Ford Escorts and other big cars: Because it's nearly impossible to make a Mini faster than cars with engine bays that could take 6-cylinders easily.
greetz
der butz
PS: I've rolled the Mini by the way... was too quick out of a corner :-)