Blue Moon,
Dont think so much about crashing. Not good for the mind set <g>. Simple nose overs are no big deal. Like was said before, not many engines survive a rock or pavement impact. So some place in the middle you might ding a pushrod tube or scuff some finish up. But I sure wouldnt worry about. I have 3 saito 120s with an average of 80 hours on each and they run fine. I do suggest you adjust the rocker arms at the beginning of each flying season and after run oil if they're gonna sit for more than 4 or 5 months to protect the bearings. The idle on mine are so low that you wait for it to die, but it doesnt. And will throttle up with no problem or delay. Durability depends on care. They DO take a little more care but you'll be rewarded with a fine running engine. The most common failures that I've seen is broken rocker arms because they were never adjusted or rusty bearings because they sat for awhile and still had glow fuel in the engine. Repair expense is relative. I can get bearings local for less than $10 each. I think the last time I looked, a bearing set for a saito 120 at the LHS was around $25 or $30. Many guys in our club just repair them or have the local engine guru fix it. I've had some 2 strokes that didnt fair so well in crashes and you just chunk the engines cause the parts cost more than the engine is worth. I used to think the same way you are right now. Glad I switched to 4 strokes and even happier I started using gas.
Edwin