Perry! How goes it man? Glad to see you here.
I sense a little bit of “not worth my trouble” syndrome with the information you received about your fireplace conversion.
What you heard was true. Ventless stoves, fireplaces, logs, etc CANNOT BE CONVERTED to natural gas. HOWEVER, there is a simple solution to your plight.
While you cannot convert the fireplace to natural gas with a conversion kit, you can convert the fireplace by replacing the burner plate. That means the whole assembly. Depending on your model, the entire burner assembly is removable by removing a couple to a few screws and disconnecting the gas supply. Once done, the entire pan with burner and logs will lift out and the new natural gas assembly can be put in. It is an expensive fix but you will essentially have a brand new fireplace without removing the cabinet or doing any demolition. The new assembly will come with new rating tags with the natural specifications as if the fireplace were always natural gas. This is the only suitable conversion for a vent free fireplace.
I don't think I have your number but let me know what model monessen you have and I will get you a quote. Your logs can be re-used if they are in good shape so let me know if you need logs too.
Stay in touch.