Honda's life span is documented at:

Top end: 20,000KM
Crank: 30,000KM

Now that doesn't mean that the parts won't need replacing prior to that, but they also could go longer.

Since it's hard to know exactly what the engine has lived with for it's life, those numbers are very general. You are best to look inside if you can, and determine where it is in its life by actually looking at the engine.

A top end will cost about $250.00, this doesnt include new heads, plugs, or power valves. This is just two pistons, gaskets (top and botton), small end bearings, wrist pins, and circlips. Then of course there is your time, about 2 hours. A full engine rebuild? Well it's hard to nail down a cost, as it determines what seals you replace, what gaskets, what "other parts", etc... However it can be done in about 8 or 9 hours (of course you will want to let the cases setup for 24 hours after they are sealed up before you start it up).
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Charles Gallant