Best way to fix that is to... not fix it.
Unless you see <30F daily during several months of the year, I would remove that hose. It is there to help provide faster warm-up, but it more of a threat than a benefit.
You could use metal foil tape to patch the hose. If you want to keep it, you need to replace the thermostat in the airbox. It will fail to only intake hot air eventually, which will destroy your AMM(air mass meter).
My advice: Tear off the whole metal preheat tube. Disassemble the airbox thermostat. Cram the thermostat flap into the warm intake hole.
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1990 740 Turbo, on its way to stock specs, maybe beyond
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