I have a 8 foot baseboard heater (federal pacific BQ815X). The heat overload part (cat.LTP 210f.) has failed and the unit will not heat. I have not been able to find a replacement part, nor a dealer who handles Federal Pacific. Will I have to replace the unit with another brand? or do you have another solution?

Replace units

I am not an expert in electric heaters but I have installed and used many of them over the years. I agree 100% with hvac1000's advice. My concern would be whether the overload protection component failed because the remainder of the unit it was designed to protect failed/overloaded and the component just did its job, or if the overload device actually was defective. Unless you have the testing equipment and expertise necessary to make a positive determination that the overload protection device failed, get rid of it as quickly as you can assuming that it is not under warranty. Unless you have already safely disconnected it from any power source, it would be very advisable to do so.