About how many miles will a well-maintained 80cc engine last?

yes it is plausible but if you want it to last for years get a 4 stroke of some sort a well maintained 4 stroke will last a lot longer than any 2 stroke kit can

There's no way to answer that. It's completely variable. That's harder to answer than asking how long a glass of water is gonna take to evaporate!

anywhere from a few hundred miles to 10k miles (some replacement parts) depends on your luck with the engine you get and how hard you ride it

I've got about 4 years of use on my Dax. I used to re-ring it seach spring but quit doing that 2 years ago. Mine is road hard and put away wet. 25% castor in my blended 2 stroke oil does the trick.

Thats like asking how long will a regular person live.. Anything can happen. Just make sure you wear a helmet because made in China is made in China ;

Same engine since 2010. No clue how many miles. Lots. Runs like it was just broken in. Even hauls my 200 lb. + butt up slight hills without me pedaling.
Now, I did quite a bit of work on it in the beginning: greasing, port matching, tuning, aligning, etc. The less hard they have to work (like anything else) the longer they'll last, I figure. I've heard some on the forum say they had over 10K on theirs.

i agree with the castor oil, its better to have too much oil than not enough, you can always clean out the carbon deposits, its a little harder cleaning up piston parts from your muffler. i got about 5000 miles on my 66cc now, reringed it once, and i clean out the carbon and varnish deposits the castor oil leaves twice a year. she still runs good. if you want your motor to last as long as possible, when you get it, tear it apart, clean it out, found casting sand in all the cases ive bought, replace the crank bearings with better quality ones, also replace the wrist pin bearing. Then follow what Allen Wrench said, port matching and what not. but a good quality oil or mix of oils will do wonders, i personally use 4 ounces Mystic JT4 synthetic and 1.5 ounces castor bean oil. a big plus, it is a widly known fact that burning castor bean oil is the smell of manliness and racing.

These 2-strokes will last for years if maintained properly!
The very first bike I ever built in 2007 still runs fine.
My GFs bike is almost 3 years old & still makes it to all the local MB rides.

moonerdizzle said:

if you want your motor to last as long as possible, when you get it, tear it apart, clean it out, found casting sand in all the cases ive bought, replace the crank bearings with better quality ones, also replace the wrist pin bearing.

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I'm sorry moonerdizzle but I couldn't disagree more.

Suggesting a rookie break open his new motor case is more likely to leave him with a broken motor before he even tries it than make it better, besides if you buy a good motor like a Skyhawk to begin with all that is already done right to begin with.
Casting sand in every motor you have bought? What the heck are you buying?

As for real gas mileage I just figure on 80 MPG because for the most part they are ridden balls to the wall all the time.
With an expansion chamber maybe 100 because all that wasted fuel that exits the cylinder is forced back in to be burned.

As for how long the motor will last that is best measured in Hours not Miles like they do heavy equipment, change break-in mix at this may hours, pop the head, inspect the cylinder and re-torque it on at at that many hours, etc.
I know that is totally unfeasible but if you really wanted to know that would be the way to it.

Treat it right from the beginning and you can measure it by worn out tire replacements and 2 or 3 times without any major problems is certainly possible.
As far as how long the motorized bike as whole will last that is as much the bike as it is the motor, I mean come on, it's a bicycle ;-}

I have no first hand knowledge of this, my personal ride is always something new but one of my very first builds 2 years ago with a Skyhawk rode by a pair of guys almost constantly for a year and half before it was stolen went through 2 sets of tires, 4 plugs, a tailpipe, and some miscellaneous other things like a clutch cable but nothing inside the motor.