Hi and welcome to TSF
When you get something this old, it is hard to really find out anything about it.
I would take all weight off the rear wheels and with it in neutral spin the wheels by hand, do they feel about the same?
Check the clutch, on the drive. I am guessing it has one rubber coated wheel that drives a flat wheel. Make sure the flat wheel has no grease or oil on it. On the rubber drive wheel, you can take some sand paper and rough up the surface a little. Make sure you don't leave any flat spots on it.