Sounds about right; I´m running a 96 model with 13000 on the clock, 50mph is a nominal top speed, though in good conditions (downhill, little wind, warm cylinder and systems) I´ve managed 65mph. On such a little engine, these little things make a substantial difference to the running. HOWEVER it should rev up to 10-11k. Check in this order your: spark plug, air intake and filter, oil and filter. Then if you have the equipment, next would be tappet heads, condition of cylinder/piston head. Use the Haynes manual for reference.