Look at it this way:
You buy the weed wacker for $200. Then you need to take it apart. Then you throw away all of the non-engine parts. Now you need to make a prop adapter.
Unless you are a machinist with access to a lathe and milling machine, you'll have to have one made for you. I used to be a machinist and I can tell you that it won't come cheap.
Finally, you'll need to adapt the mounts to fit a firewall (Back to the machine shop).
Or... you can spend another $100 and get one of these, bolt it in and you're ready to go.
http://www3.towerhobbies.com/cgi-bin...&I=LXNRH4&P=SM