the engine fits. But the mfi has a very complex wiringloom... you need to cut the MFI (engine loom) off the loom from the ERST and get it in your Si. Only thing you probaly need to mod is the radiator to fit the intercooler...

also you need to modify the downpipe. And maybe the fueltank.

a easyer mod will be to fit a FRST engine. As it is straight swap.