in my vast experience of dealing with stubborn old rusted shite
.. once it don't come off straight away, with reasonable methods, then drastic action is needed,
probably gas torch
but it is difficult to get at usually, and may need stripping down or engine removal, not to damage other bits.
Especially so exhaust nuts or bolts.
my old TTR600 .. exhaust nuts .. twat ross morgan from motopro busted them off deliberately, after supposedly trying `everything` and I had to pay an engineer to re-drill 4 holes and new studs and nuts .. and another twat glen philips of gap motorcycles to bodge the head re-fitting to give me back an oil-gushing leaky engine shite hell. (no bitterness there then
)
ps ... these 2 twats cost me well over £1000 and gave me shite
.. so in future, no more back street `mechanics` to `save money`
.. I go to a proper mechanic now
(and now got new bikes for `easy to work on`)
so my advice .. if it don't come off straight away or after a reasonable effort ... go to a reputable trusted mechanic ... it is cheaper in the long run.