We have a 15 year old briggs powered 4500 watt continuous generator that was originally only gas but we added a propane/natural gas conversion kit easily and cheaply that still allows it to be run on gasoline.
You might be better off to get a generator and adding a conversion kit to it that still allows you to use gasoline. That might limit you to machines with the big engine manufacturers (briggs Honda kohler etc). Many generators these days have Chinese engines on them that parts and aftermarket accessories aren't as readily available.
We got our kit off ebay. You search the kit by the engine model, not the generator model.
Just another option to think about.