I've got about 1/2 way.
Made a manifold with 4 injectors, made the fuel rail, got the HP pump etc plus sensors. Got a Rover EFI diz for modification to crank angle sensor. See my set-up:
I've made the intake runners as long as possible for max torque but still large for high HP.
As stated above, the 2 ports per cylinder and firing order means that with standard group fire EFI, two cylinders will run rich and two lean. To overcome this you need to use a lower flow injector with very high duty cycle (electric carby like Rover SPI) which gives an even air/fuel mix throughout the intake, like a carby does.
The other solution is to use sequential injection and time the injector pulses just right so that each cylinder gets the correct mix. The Rover MPI is sequential. Make a change to the engine and you need to re-calibrate the injector phasing. The Rover MPI system also aprays the fuel onto a large surface area (combed) part of the manifold. This has the effect of evening out the air/fuel mixture during the intake stroke as opposed to the sudden changes that occur as injectors are open for only a fraction of the stroke.
To do a multi-point system for a 5-port mini you must have a computer that can do sequential (eg Haltech and Motec). You must have a crank angle sensor system that can pick No 1 TDC at Firing stroke against exhaust stroke. This means you need a sensor on the cam or dizzy, not just crank. Hence the Rover MPI cam sensor in the back of the engine and my design of sensor inside the locked Rover dizzy.
Then you'll need to put two oxygen sensors on the engine to tune it so that you can get the A/F ratio right for inner pair and outer pair. A single sensor will just give you the average. Could use 2 exhaust temp probes instead to balance and a single o2 sensor.
Then you need a tuner who understands what the engine needs and can use the injector opening delay parameter and sequential functions of the computer to set it up right on the dyno.
So, the physical stuff like I've done is easy. The computer stuff is easy to install but expensive to buy and tune. My project is stalled waiting until I can afford it.
M