Be darned careful about putting any kind of enclosure around an air cooled engine assembly. If you keep all the sound in, you may keep all the heat in also and you will end up with an expensive crispy critter.
I was at an RV site the other night. 5KW RV unit brand new is about $3000, Used should easily be less than 1/2 that. Also, all the RV style ones are electric start - that's a nice feature.
If you were to use a car-sized muffler, you would want to leave the stock one in place to generate the proper back pressure for the engine.