Natural gas to Propane change over help

About 20 years ago I was given 2 Reznor heaters for natural gas. They were purchased for in a fire house and when they remodeled they went with a gas furnace. I used one of them for in my 20x24 open 14' peak ceiling shed/garage with great results. The other one has sat in a corner since then. They were only used about 2 years at the firehouse which would be very few hours on them. The one i'm using here at home has been in use for at least 15 years with no issues.
My second garage (which is my dads) is 28x36 with 16' ceilings. I dont work in there that often and I'm currently using a 125,000 BTU kerosene torpedo heater when I'm there to heat with. But it's so noisy and my dad hates the smell and noise. So i thought I could use the 2nd Reznor and convert it to propane. Was wondering if anyone could tell me if the BTU's are good enough to heat that area and also where I could get the conversion done. I dont want to do it myself as I know nothing about it and wouldn't want to risk a fire or explosion.
The model on the Reznor is XL105-5, the sticker on the side says:
normal input 105,000 btu's
Thermal output capacity 81,900 btu's
Serial number is AMJ31G8N64843

I've read that some of the older heaters can not be converted which means I would need to update to a modern heater. If so what recommendations for brand do you have?

Thanks Rod