In my opinion, the real problem with these conversions is the flue/chimney arangement. Flues for wood burning are bulit quite differently than those for gas burning appliances. A gas flue will not work well (and may be dangerous) for a wood fire, and a wood flue will be inefficient venting for a gas fire, as Peter has said.
This applies to the TOTAL REPLACEMENT of a wood-burning appliance with a gas-burning one. The idea of fitting a gas burner into a wood-burning firebox of a wood-burning appliance is another matter altogether. Such modifications are extremely problematic and unlikely to succeed.
NtP