In my experience of long flights the Percival Prentice was quite good; rumbled along on cruise RPM/moderate boost, very stable, all the way LGW to Sharjah, quite a long time ago. Some downsides, like the occasional quietness when the engine stopped in cloud because it didn't like dampness or caught fire, bit slow, drank fuel, but in general not a bad aircraft on a cold day. There's one at Coventry.
Mind you, that's my only experience of a long flight in a light aircraft. A grown-up spamcan could be boring, I think. What about one of the "microlights"? Now that would be fun. 450Kg for you, fuel and the aircraft. Should be enough. Mogas, too, I think, on a Rotax?
Think outside the box!