"Once you get away from the certified engine scene, there is a huge variation in engine reliability."
May add my few pence worth? I've clocked up almost 1200 hrs in the last 19 years, all bar an hour or so in the same flexwing I learned to fly in (which is possibly a record in itself?). The only time the engine has stopped in flight is on the few occasions when I have turned it off.
I have experienced 2 instances of fuel starvation, due on both occasions to water condensation mixing with the oil in the fuel (50:1 premix for the Rotax 447) and partially blocking the fuel filter. The first time was during my GFT, and we managed to limp back to the airfield. The second time I happened to be passing a suitable strip, and immediately turned round and landed safely.
More recently I have suffered several misfires during climbout - on all occasions the engine recovered after throttling back. This took me some time to resolve, and eventually turned out to be the original engine wiring harness. Since replacing, it's run perfectly.
When I hear the experiences of some other pilots who regard engine failures as a regular occurrence, I have to wonder why... Maybe years of motorcycling taught me to take care of 2 strokes? I only use good branded Mogas from the same garage, and buy my oil in bulk. I don't take off without allowing the CHT to reach 200F, and I never throttle right back on approach until I'm assured of a landing - no fuel = no oil... My machine has a simple permanent warm air induction system which seems to be enough to prevent carb ice from forming, without noticeably affecting performance, another reason for keeping some power on during descent.
This goes against how I was taught, but I'm of the opinion that glide approaches are asking for trouble on older 2 stroke powered machines like mine. I do all my own engine servicing and rebuilding, and keep to the recommended decoke intervals, so I know I won't have any problems undoing bolts or removing gummed piston rings, etc.
Harking back to some of the earlier comments I'm not constantly looking for somewhere to land - if I really was that unsure of things, I would have quit long ago. I'm supposedly flying for enjoyment, and I take the view expressed by others of trying to minimise the likely hood of engine failure in the first place.