Your question about the exhaust might be interesting. I had an air-cooled 125 beta when I was younger and after two years of practise it didn't run properly, I mean it had no power anymore. My Dad looked for an explanation of that lack of power for a quite long time until the day he wonders about the exhaust. But I'm talking about the central part of the exhaust, not the final one exclusively. The central part of the exhaust is the most important part and if it's full of non-burnt oil and that kind of things, the engine's power is lowered.
To clean it, if it's not in aluminium, there's no problem, make a barbecue !! Not kidding, make some wood burn in a barbecue and put the exhaust in it. It has to burn slowly but for a quite long time in order to make the oily paste which is in it turn into ashes. Then you just have to repaint it.
Now on the new bikes, we couldn't do it anymore as the central part of the exhaust is in aluminium... It's more complicated to burn. So I just have to "clean" my bike by riding back home by the road, gas full open !!
But don't forget never use any inflamable product in order to accelerate the process = danger !!