Thought about a Chinese Yanmar diesel engine clone?
e.g:
http://cgi.ebay.co.uk/YANMAR-TYPE-ENGINE-UD178-DIESEL-NEW-BARGAIN-PRICE-/260693256592?pt=UK_BOI_FarmingEquipment_RL&hash=item3cb2876990Available from 4 up to about 10 BHP.
The electric start models have an alternator built into the flywheel.
Max RPM is 3600.
I believe you can get a model with power take-off from the camshaft so 1/2 speed.
I guess you'd have no reverse though. :-/
Lots of generators with these style engines come up on ebay so you could probably wait for one with fried electrics and get the engine relatively cheap.
I bought a knackered one 'cos I fancied a project a couple of years back (got a project too - it had been run without oil). Anyway, it did a 24 hour radio contest on about 6 or 7 litres of diesel so pretty frugal.
Not quiet, mind. You'd want to box it in on a boat if you want to be able to hear yourself think.

Kevin