For black iron pipe it's called a floor flange but the pipe screws in rather that uses a set screw. So maybe it's called a ladder roof flange. I can't recall seeing any loose parts without buying the ladder except at RV surplus stores.
I replaced two ladders on my neighbors 5th W in the past year, yes he backed it into his shed twice. Not sure of the mfgr. on all of them but the second was a stromberg carlson brand and all three had a screw that came up through the bottom of the flange and screwed into a threaded part that was pressed into the ladder tubes.
I searched on roof flange and didn't find anything either.
Reuse the old one and cover it with sealer.
edit: I found one for a 1" tube:
https://www.globalindustrial.com/p/storage/fittings-framing-rail/Kee-Klamp-Fittings/aluminum-flange?infoParam.campaignId=T9F&gclid=EAIaIQobChMI25qIwKi42wIVCb3sCh1SqA0uEAkYDiABEgLQtvD_BwE
There's also some 3/4 hardware on that page.
