Tho admittedly not without it's own shortcomings (price & range), I'd hafta say the absolute most
"quiet & reliable" method of motorizing a bicycle has got to be electric...
..but given yer "cc" reference, that's prolly not what yer after lol, then again what you've asked is going to be a bit controversial as everyone has their own favorites. I'd suggest perusing the various subsections & seeing what others have chosen, you may wish to click some usernames and view their previous posts & threads to get an idea of their experiences.
Welcome to the forum scottsbone, sorry for the sorta "nonanswer" but tbh, there's such variety, so many different "tools" for different needs, so many options it's often simply preference