Is my mower worth fixing?

I have a 92 John Deere F510 mower with 760 hours and the Kawasaki motor on it. Well it has started blowing smoke after running for about 45 minutes it will blow a big puff of smoke and loose power. You let it sit for a couple of hours and it will run another 45 minutes. I think it may be the rings, and it is the motor with the hydrostatic drive built into the motor.

It has not been a good day for mowing. My gas powered weedeater bit the dust. After that I went to the old electric weedeater, and it lasted about 10 minutes before the guard came off of it.