Don't go assuming that the jetting is too rich ... unless you are real familiar with 2 strokes.
It could be too lean because the carb is clogged.
What does the spark plug look like when it is all warmed up and this bogging is taking place?
If you just bought this bike... then it is a good idea to do a slow and thorough carb cleaning like it says in my "Troubleshooting and Tune-Up" post below.
THEN see what it runs like.