I pulled this motor out to rebuild the top end, but when I got it apart it wasn't as advertised. Cylinder is clearly marked as a 110 on the outside, but is definitely not inside. I'm assuming it's just a factory error and is probably a 70cc. I still want to rebuild it, but the only pistons I see are 42mm and the cylinder only measured out to 41.5mm. Should I go for the 42mm and hope the measurements are rounded up?

 

I'm always a bit dubious of using verniers to measure things accurately, however it does look like 41.5mm bore. If you can find the engine number, this has the bore in the code, starts with a 1 for single cyl, bore dia next, though this would be rounded to 41 or 42, then FM for the engine type. Looks like the maker has used the 110cc casting for every model, and bored it to suit whatever engine size they wanted to make. If your piston is beyond re-use, you could bore the cylinder to 42mm but be sure pin dia and crown height, etc. are the same on the new piston first.