Don't know if am too late to reply, but i put a 1242 in a 899. Just use existing loom all the connector is on there. And you can just add a wire to

black wire near the ECU. (this is documented on FF look at the guide section). Here is just a list what you need to change:
* 1108/1242 engine and gearbox + all engine mounts
* Exhaust system from the sporting
* Gear linkage from sporting
* Thermostat from sporting
* Starter/alternator from the sporting (1108)
* Header Tank from sporting,
* All Radiator hoses
* Drive shaft from the sporting
* Cannister from sporting
* dial clocks from sporting (add a wire for rev counter)
* speedo cable/ clutch cable from sporting
* 15mm spacers for the exhaust mani. (taller engine)
This is the list i made up for the 899 to 1242.
Ming