When you say matte finish, are you talking about the black parts, such as the engine covers, etc...? If so, try spraying some WD40 on a rag, wipe it around - it's really amazing, it cleans and makes the black really black again. Just don't leave it too wet, keep rubbing with a dryer part of the rag until you take most of it off. I've used this for a while, and it works great!
2012 Jet Black Bonneville T100
1970 Suzuki T350 (2-stroke!)