maybe a daft one but..........re weld the flange?
Those studs look like they are well rusted so expect a breakage, don't do it when you desperately need the bike.
I have seen this happen recently, It sems common with aftermarket stainless pipes. What make are they?
The original downpipes were black and will have corroded through well within your bike's age. They do look very similar to mine which are Delkavich and broke in the same place.
Dont try just wd40 youll waste alot of money if the studs snapped (happened to me, luckily i have good friends)Get the rad off, get a propane blow torch and get some heat on them
Of course it could still snap the thread clean off with the nut despite the propane. Regular coppaslipping of the studs and nuts to avoid a similar situation if i ever want to take the exhaust off again i think...