RE: What to do with a good 25cc WeedEater engine?
For a period of time Zenoah was providing the engines used on stand up scooters. That engine was originally a G-23 but was later changed to a G-26 when the 23cc engine was discontinued. So a 25cc engine would work fine on a scooter.
Your other options are to turn it into either an aircaft or boat engine. A bit more work and parts required for a car engine.