Putting a plug socket on a cable which was previously wired into a heater?
SMH how is that simpler?? 

You get a new heater of the same basic form-factor, bolt it up in the old location, and connect the wires to it like it was to the old one. 

Clearly the skill to attach wires is not absent, since you plan to wire it to a receptacle.  

Do you possibly have a mental block about "gosh, I don't know where to get one of those" or "surely they must be expensive"?  Home Depot, and $50-100 depending.

I know it's not at the top of your list of things to spend $100 on this week, but life happens.  You'll be getting something that'll last 20 years, as opposed to those little plug-in heaters that won't last a season if they're your primary heat. You could easily spend more on junky little heaters than you do on the proper one.  

Further, the proper one is surely 240V -- and that means you'll need to use a NEMA 6 receptacle, and you'll be paying a lot for weird heaters. You could buy a $50 Cadet made-for-permanent-installation heater and put a cord on it, but why not just permanently install it? 

At best, a plug-in heater isn't going to be cheaper enough to justify the trouble.  Might as well just do it right.