I'm guessing you mean the kit by Minisport?

Hard to make out what it is they are supplying for a cool £5600.

Looking at the photos this cost is for the basic engine cradle kit.

Headers, gear selector, slave cylinder, reverse motor, cooling hose & pipes and axels appear to be an additional £4000.

got an email today from another mini parts supplier.  They talk about a R1 conversion kit for your classic mini.  Interesting to see.  And it doesn't need a different sub frame...not cheap though

One of the key advantages of our R1 Conversion Kit is its ease of installation. The modifications required are so minimal that you don't even need to remove the subframe from the car. This also means that it's easy to switch back and forth between the 'A' series and R1 engines, giving you the flexibility to adapt your Mini to different driving conditions or events

To complete the conversion, you'll need our kit, a classic Mini, a Yamaha R1 engine, Yamaha engine wiring loom & ECU, fuel pump, air filter, Yamaha clutch cable, and a gear change system. We also strongly recommend upgrading the brakes and suspension to handle the increased power.