Spare tire mounting problem. HELP please

I am new to owning a Mazda truck. It is a 1990 and the spare is laying in the cab behind the seat. It is the extra cab. I want to mount it under the bed like it used to be but the mount is rusted. What are my options? I do not want the tire in the truck. I would rather mount it under the bed. Is the an easy or inexpensive solution to this??

I am going on memory for this.I am sure the mechanism is bolted up and would just spray LOTS of penatrating fluid on the chain and or bolts and let sit I mean coat it.If it is bolted up(sorry for not chkg my own trk) but lubing these normally will release it and make the plate with chain drop down.I might look over mine wed and see if I am forgetting something.I normally drop my spare every couple months and lube chain and or gears and my spare is so dry rotted and orig.Just thought.I am going fullsize tire and leave in my bed which is covered reduces moisture and easier to access.

You could get a non rusted one out of a truck at the junk yard and swap the units. It bolts in with 2 or 3 bolts from the top side of the tire cross member if I remember right...

My 1988 had a broken link in the chain when I bought it in 1994. So I got one from a junkyard and bolted it in. I now have a full-size spare on it under there.