Your problems are not the internal mettalic materials. Your issues will lie
on using a thermocouple rated for your expected EGT and reprogramming the ECU for the specific energy of methane, and pbly improving insulatiin of components inside the cowl.

You need help from JetCat for sure, and that comes at a cost. I would start with an RFI/RFP to JetCat or Dreamworks RC if you are serious.

David