Hal. Bristol owners have tried all sorts of transplants, and from my viewpoint, one of these days a Bristol owners contingent should approach Tony Silverton at Bristol Cars and ask that the factory come up with an "approved" solution. Unlikely that it would be remanufacture of the old Bristol 6 and transmission, because the cost would be over the top even if they had it made in China.
More likely something along the lines of what they did with Chrysler, where they are phenomenally lucky to have chosen a power plant that was in production in 1960, and remains, in almost the same form today in 2010. However, it probably needs to be a lighter engine, and I envision Bristol developing a retrofit kit with the ideal making it a bolt-in solution using a common engine with similar performance characteristics to the original. Probably Japanese if they want practical, but could be German if they want tradition.
Having said that, there may be another direction for you to go. Electric. Check out the electric car conversion business, and you find it is not all microcars. Ford and Chevy made pickup trucks that ran about 100 miles on a charge. The motor part is easy and small - it will fit into the drive bay, and typically uses a manual transmission. The battery part is the weight and cost, but a lot of this depends on range. Once installed, maintenance is far less.
There are companies that will do it all for you, if you have the checkbook. See
Electric Blue Motors Conversion Services for an example, quoting about $13,000 for a car. Do it yourself seems to be around $5,000 for parts.
A few years ago, this would have been considered an off the wall idea, but times are changing thanks to folks like Tesla. The fact is that most classics are not driven over 100 miles at a time, and if you do take the long outing, a gasoline powered battery charger can extend range. The Bristol can store batteries in the two side wings as well as behind the rear seat where the gas tank goes without taking space from the passenger compartment or boot/trunk. With the side wings you could probably design a quick remove kit, so you could keep a batch of fresh batteries in the garage and swap them out in seconds.
And, if you are looking for cheap batteries, a friend of mine put a wind generator in his home on a micro-island in Rhode Island, and he bought a room full of used batteries from a nuclear power plant. Apparently they are required to replace them on a schedule, even though they have plenty of life left in them.
Just another idea to throw into the mix.
Cheers
Claude