This rover siamesed port charge robbing problem is a very difficult thing to get around.
I have actually studied how rover get around this and it IS a pain in the arse
to set up.

You will have problems using an aftermarket ecu on this engine type using
standard multipoint injection techniques.
Especially on a simple system like megasquirt.

There are companys out there who DO remap the rover ecu's but I dont
have any details.

Unichip may be an alternative .....