Fuel injection on 5 port mini engines is pretty tricky to get right due to the siamese ports, and for a carburettor car you need to add a new fuel system with high fuel pressure, return lines, all the sensors to get the ECU to work and you'd add in a new ignition system at the same time.
There is one here that had a similar weber style injection with a downdraft setup. You might need to register to view the posts.
http://mk1-forum.net/viewtopic.php?f=6&t=7090&start=130He also does some videos on youtube
https://www.youtube.com/watch?v=WYiNOAEn96Q