Hvidt & Molgaard Minerva sofa back frame only  

If you don't want to wait for a damaged frame sold as parts, your best bet is probably to have one made. It would mean determining the dimensions of the parts and finding a cabinetmaker and metal fabricator who can do the work. Looks like the metal parts of the armrests are the same as on the back (except for being bent and being shorter) so that will make it a lot easier. Good luck!

I agree: do the homework to figure out the exact dimensions and have it made. This is a nice enough piece of furniture I think it deserves to be made whole again. You can get the teak from eBay. It generally comes from Costa Rica, so be sure to get pieces that don't have the whitish grey sapwood in the finished piece.
The likelihood of finding a spare backrest is next to nil. You would probably have to buy one of these sofas that got completely trashed in 'as-is' condition to get the backrest. And good luck finding that.

I'd like to see it re-invented as a day bed.
I would,
Aunt Mark

At least insofar as a daybed has a clear definition, I believe that the Minerva is a daybed....