I recently damaged the engine on my Honda mower, so here is my experience (Seattle area):
The engine is well built and fairly easy to work on. Their manual specifies a bunch of special tools, but I was able to take the engine apart and put it back together with just "ordinary" small engine tools. The only problem I had was with the flywheel. It has no threaded holes, and needs to be held around the periphery with something like a belt-wrench while torquing the center bolt. For the same reason, a large gear-puller is required to remove the flywheel from the crankshaft. I did not attempt anything ambitious like replacing valve seats, though. All threads on this engine are metric, of course.
The manual (available from dealers) is very clear and well illustrated. Carb rebuilding is not covered, however. Part numbers are listed in a special "parts manual", not in the regular one.
Note that Honda appears to have a policy of not letting anybody but their dealers get in touch with their central tech support. They don't publish their phone number, and there is no email address either. I could not find the timing mark on the crank, and the dealer could not or would not help, so guessing was the only option left... I don't know if other small engine makers are any better in this respect, but I am used to customer support meaning something entirely different in my line of work.
No trouble getting parts, but they all had to be "special ordered" from out of state, taking about a week to arrive. They seem very expensive to me. A rod was $50+, a set of piston rings, valve springs, or head gasket $20+ each, and the crank (which I luckily ended up not needing) was supposed to set me back about $185. My relatively minor repair ended up being about $125 in parts (taxes, oil, and band-aid for bruised knuckles not included).
The mower has served me well, and the only problem other than caused by my own fault was premature wear of the blade clutch lining. The plastic deck seems to take rock hits much better than an aluminum deck that I owned some time ago, and there is no paint to peel of from it.