RE: weed eater engine
a good 60 cc chain saw engine can put out 10 hp without a pipe, we have a husky 60cc that is 12 years old, never been rebuilt and still runs 17:1 compression, if your looking for a good engine they are they way to go, the only problem is that a good chainsaw can run into 1 and a half or 2 grand (our husky was $3500 AUS 12 years ago) so in that respect a purpose built RC engine is the way to go, but chainsaw engines have a lot of potential once ported and have a good tuned pipe.
also if your looking into around 60 cc engines, the Kawasaki KX 60 engine puts out 16 hp, and a KTM 50 engine puts out 13.3, the 50 uses a simple gear reduction box and centrifugal clutch so if you wanted, you could mount that in an RC with some custom engine mounts and some modifications to the given car/trucks chassis to allow for it's larger size.