The plug is not manufacturer installed, also the heater is a permanently installed unit.
If the heater was manufactured with wheels or a stand and a cord with plug you would be OK and could utilize it that way as a portable unit.
Now, if you so chose, as the home owner and accept all liability, to install a plug & cord onto the heater. Or chose to hardwire the heater in and leave the plug usable and relied on your common sense to not use both at the same time.......Let?s just say I have seen worse home owner wiring jobs.
I cannot condone or advise you on doing the above mentioned methods, what you do in the privacy of your own garage is your business.....
Now pay your bill, or I will start a thread and out you as a dead beat !