Just to flesh out John Farley's comments:
A good rule for low speed unswept wings is that lift curve slope is proportional to AR/(AR+2); in the absence of anything better to a good approximation you could use this scaling for the CLmax also.
The Gottingen 535 data shows a sectional (2D/infinite AR) CLmax of about 2.25. Scaling this to an AR of 4.5 gives a CLmax of 1.56.
With 800lb and 111 sq.ft wing area this would give a stall speed of 62.3 ft/sec (68 km/hr or 37 kts).