So, the molding around the inside of the door looks applied (not part of the frame) to me?
If you can cut enough off of the ends and sides to get it to fit, I'd say go for it. Worst case you're out $5 so no great loss and we'll all learn something more about door construction (as long as you share the insides of it with us
).
If that's not possible, pick an end, probably the end with the narrower stile (cross piece on the ends) and see if you can pull up the molding and see whats under it. If you can remove the molding (it might be glued down so you can't in which case.. carry on).
Cut through the side rails (the up/down pieces on the sides) a bit longer than you think you need. My guess is that the panel should be free floating at this point, if not then its a bit more complicated.
If its as I suspect there then we'll want to take a look at how the stiles are currently attached to the rails, ideally you'd be able to replicate as much of that as possible likely with the addition of either some dowels or a floating tennon (basically you cut a slot in each piece and another piece to fit and glue them together). When you get closer to this point check back in. You can do all of this with hand tools (a chisel and a saw basically - I've done thing not unlike this when broke and in college and needed to "make it work"..) or a hand drill for some doweling - which might be a bit easier to handle to start with.
So clean up the inside of the stiles and remove the remnants of the old rails and figure out how to fit them together. Cut the rails to length/match and assemble.
If the molding was removable, cut to length/fit and re-apply.
Use a good outdoor finish - like spar varnish and then hang. We'll skip hanging the door for now, that's its own set of tricks.
If it looks weird/wrong/confusing at any point please do check back in - the crowd here is good and willing to help
An outside door isn't a super easy first project, so don't get to upset if it doesn't work out exactly as hoped for.. but like you say $5? what have you got to loose! It should be a fun and interesting learning experience either way as long as you don't mind hacking it apart and seeing what happens
.