Natural Gas and Propane appliances like your heater can be converted from one to the other. I would not run propane in it unless you convert it back to propane first. Here's a little information on it: http://www.propane101.com/lpgasapplianceconversions.htm

Excerpt:
The conversion of any gas appliance to another fuel involves not only replacing the orifices (fixed and pilot), but the replacement of appliance regulators, burners and possibly the venting as well.
If you have a gas company locally, they may be able to help you convert it to propane if that's what you want. Personally, I'd probably look into getting some natural gas and using it with that.