At 21lbs without the engine, you are looking at probably 26-28bs ready to fly- assuming you will be adding engine, ignition, battery, prop and spinner.

I would say that the 100cc class engine is the right engine for this plane- DA, BME, ZDZ, 3W are top of mind. A ZDZ 80 would work fine as well and is more moderately priced.

As far as keeping it quiet, - use an cannister muffler (KS, JMB, etc.) and, perhaps a 3 bladed prop. If you can get away with a 2 bladed prop that meets your noise level needs, that's even better (better performance, lower cost)- the MSC props seem to work well for this- they are beefy, turn up a bit slower and still give good performance.

Dave Michael