How do I limit the maximum power, delivered to a heater, through a triac?
If the existing heater is a fire hazard, you must select a different heater! It is far too easy for some part of your "regulator" circuit to fail in such a way that full power is applied to the heater.

One simple solution would be to wire two heaters in series &mdash; each heater would operate at 1/4 power, producing lower temperatures, and the total power would be half that of the original heater alone.