Many of the LPG burners will run on NG just as your average BBQ will - BUT you do need to upsize the jetting. If you bought your burner at a BBQ shop, go and ask if they can supply the NG jets. DONT try and drill the jets yourself as the hole diameter is very critical.
If you can have a look at your existing LPG jet it should be marked with the hole diameter - something like "0.8" or "1.1" depending on the burner rating. I can look up the jet calculator wheel and find the appropriate jet size for NG so you know waht to ask for.
You may also need a new regulator for NG although some burners will run straight of the NG mains pressure (both my Carmichael burners run direct on NG)
Wes