Good engines from here: http://www.motorizedbicycle.ca/
they sell grubee engines which is one of the most popular engines out there.
As for dampening sound. You could try putting something outside or inside the clutch cover. Some people got good results while others including me isnt really happy about it.
Here is an example: http://www.youtube.com/watch?v=GUZcD5Cpe2I