From the book "Driving Polski FIAT 126p" (Warszawa, 1982):
The trailer's total weight, without breaking system, can't be more than 50% of the weight of the car itself - 300kg. With breaking system it can be max. 400kg.
With caravan - top speed a bit more than 70km/h, fuel consumption with 30-40% more depending on caravan type.
If bad weather or slight climbing should run on 3-th gear.