Thanks for the replies chaps.
It seems like there are presently lots of difficulties in this idea, although all do seem to be things which could be worked around, if the investment was there.
As achimha mentions, it appears as though aviation technology is stuck in the previous century in terms of engine design. Maybe plonking a car engine in an aircraft is not such a good idea, but there is so so much technology, which has already been developed, which could be adopted and incorporated into GA engine building.
Which GA engine builders are on the front line in terms of R&D?
Are there any exciting offerings currently being worked on, or recently released?
Simon.