Hi Nigel
The M52 engine requires a matching ECU, immobiliser and key which is not too much of a problem...however, I have found that this is not enough to maintain the rolling codes for very long, as they all communicate via the instrument cluster as well.
My attempts at wiring in an instrument cluster after doing the rest of it I thought had gone well, except that the same problem of getting about 50 starts and the immobiliser cutting in recurred.
My Sportster is currently in the hands of a BMW wiring expert in Caerphilly, he has wired up another loom to solve the problem, except it hasn't, yet....
Hopefully if you go down this route, forewarned is forearmed
Although keeping as much of the donor loom and instrument cluster as possible would give you a much better starting point than I had.
Hope this helps.
Tim