If the exhaust is literally broken then just replace it with a similar looking one, there are plenty of imitation exhausts that act and perform the same as the original with decent quality. This will save you the trouble you will have jetting the carb and adjusting the rollers and such, when buying a 'performance exhaust'.
The original (or similar imitation) exhaust will give you almost the same speed and acceleration speed as a 'performance exhaust'.
About the airfilter, only replace it with an identical one or imitation, but must be a ' big bog ' again, dont get a "powerfilter"! That will just suck in a lot of dirt and require rejetting of the carb, which is usually harder than with the original airfilter. If you just mean you want to replace the foam inside the filter, then thats no problem, and doesnt matter which brand you take for that; its just a piece of foam
Be sure to get plenty of oil in your machine, also replacing the sparkplug can never hurt. Buy the same one as what you take out of it
When the time comes that you really want a lot more power, then you can look at a 70cc kit, but thats not something you will get done in an afternoon. The above mentioned is.
Oh and also check the rollers to see if they dont have too square sides. If they do, measure them all 6 and divide the weight by 6, then buy new ones with that weight. Or read the weight on the sides of the rollers, if still readable