1991 BMW 525 flush out stop leak

The patient: My friend’s 1991 BMW 525
The symptoms: Overheating, no heat in the cabin, and radiator stays cool.

The story: Picked this up for $1000, advertised as having an overheating problem. The previous owner had the thermostat and the radiator replaced, but it still overheated, so off it went to my friend. I advised that given the play in the waterpump that it was due. So out that went. Got it bled out, and ran fine at highway speeds, but would overheat in traffic. So the faulty engine driven fan was replaced.

However now there’s no heat in the cabin, and the radiator is staying cool. I decided to take a look at the thermostat and I discovered it was all gummed up with what looked like stop leak. There’s no oil in the coolant, and no coolant in the oil, so I suspect that the previous owner’s use of stop leak has clogged up the radiator and the heater core, causing the overheating.

So my question assuming this diagnosis is correct is will a radiator flush do the trick?