The frame horns tend to rust from inside out - plus they get twisted from larger engine torque. Once engine is out (and trans since its being replaced with a new one), I'd tap the horns to see if they are rusted through or just bent (suspect combo of both). I've not seen new OEM style horns available - just aftermarket one (as shown in photo). Might be able to straighten and weld on steel plating if just bent and add a Kafer bar for support.
Here's a VW rear portion I bought in VA Beach for the trans and chassis id/title - it had a badly rusted thru frame horn. (Ha, other side was ok from engine oil leak)