There are many many different types, makes and models. But if you are a true enthusiast, it has to be one of these a National:
Original from the 1930's..
Restored National DM2 - YouTube
They will happily run on used engine oil and are engineering masterpieces that will run for many many years.
A company called Russell Newbery will build you one now for around £16,000.
[ame="http://www.youtube.com/watch?v=_Q2jdanJkYg"]Starting a Russell Newbery - YouTube[/ame]
Similar engines were made by Kelvin & Petter. At one time Britain was a prolific producer of these "Donkeys" many used for stationary generating sets and pumping duties. They would run for months without attention as long as they got any combustible type of fuel,
any. (Except gasoline of course - it would have to be 'heavy')
They are much sought after and the engine of choice for enthusiasts, however, for practical and economy reasons, my sister has a modern compact diesel engine. True to form, she doesn't know what make it is (or cares for that matter).
Really, women!
Mark she took delivery of said craft 1 year ago exactly and it is a lifelong dream of hers. She is 71 years old and was beginning to think it was never going to happen, but here she is. Just proves you should never ever give up..