The Peek is designed in these extruders to operate as a thermal barrier between the hot end and the extruder. If you use aluminium to replace this piece you will need to come up with a way to cool it to a temperature compatible with your extruder body. Trouble you have with this is that Alu conducts heat well. If you cool the Alu, heat will transfer from your heater.
Not impossible to get round the issue I am sure, but probably you will need to redesign the extruder to take into account the heat conducting powers of your replacement material