Before this gets too involved, be advised that replacement parts are NLA on these.
I have one right now that we're getting ready to remove the entire fireplace and replace it. I have another one that the manufacturer has a retrofit kit available.
Honeywell fireplace parts are pretty much NLA at this time. Some manufacturers have kits made to switch out the gas train, usually to SIT.
CSIA Certified Master Chimney Sweep
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