Its not worth putting an MFi Engine into an XR3i That came with factory fit EFi to start with.

The MFi engine can be converted into EFi very easy by using a conversion plate on the head and an adjustable trigger setup on the crank pulley.

The CAR loom you have in the EFi XR3i can be used to run a FRST ECU and sensors.

SO:-

Conversion plate
Crank trigger setup
Management system, ECU, MAP sensor, ACT sensor.
EFi Inlet off any CVH engine.

Obviously the need for the rest of the running gear like T3 and zorst, rad etc..

Regards
Jano

(I have a sensor if you have problem finding one.)