Neil,
It is really up to you.
The Fiat 2 litre twin cam engine really is a classic , in design and performance and would be my automatic choice every time. However it is an older unit , the spares are getting a bit rare (although pattern parts are available in Europe). It is a true "sports car" engine that loves to rev and suits the Morgan well.
The killer might be what about Australia , and here I have no idea at all. Were Fiats ever imported into Oz? If not you might find it rather difficult to find spares or even a channel to import them.
The Rover engine had 2 iterations , the M16 and T16, the latter being the preferred option. Originally these engines were a mite "unloved" by enthusiasts as they were not really tuneable, they had a poor reputation for reliability ( but I cannot recall why, perhaps others here have more info) and they went out of production pretty quickly. I think the tuning issues are now resolved as are some of the reliability points. However for me the key issue is the match to the car. It was designed as a saloon car engine , and it is not a free revving flyer (unless tuned).
Again I have no data regarding spares etc in Oz , I would think it might be easier than Fiat , but I may be totally wrong there. I would advise you to get in contact with the Australian Morgan clubs and ask their advice; there is at least on regular contributor on this forum.
Just my two pennyworth.
