It is also pretty rare to get 15 gallons. People have gotten this far with an engine. But most motors will only last about 3-7 gallons, depending on how they were treated.
For a rebuild you will have to replace the piston, sleeve, and usually con rod(just to be safe). The con rod often gets a little streched over time and use. Although not required for a rebuild.

But an alternitive to rebuilding an engine is getting one "pinched." This is where they apply even pressure around the outside of the piston sleeve, This shrinks the inner diameter just enough to regain compression.
You can use the search button to do some more research on RCU about sleeve pinching.