Hi. Just a wee thought for you. Patience. If old pipe is completely gone, then no need for engine out.
Penetrating fluid all around bracket holding pipe.
Next use hacksaw blade to cut old pipe top and bottom of bracket. Just wrap hacksaw blade with plenty of tape and take time to cut of old pipe.
More penetrating fluid.
Now you should be able to get mole grips onto small bracket. Just try to move it remembering down helps more as you are trying to loosen allen screw.
Your just trying to loosen bracket between allen screw and head at this point. Even afew taps with long screwdriver but gentle taps.
More penetrating fluid.
Now hopefully you have this or borrow one. Dremel drill with small cutting disc. Just take time and cut slot in head off allen screw, for flat head screw driver.
Now all fingers crossed it should come out. Again penetrating fluid and a few gentle taps can also help.
You can use a small amount of heat but only small as the allen screw head at this point wont take much heat to break.
Failing the above ,, much the same ,, but just remove down pipes and youll get all access needed. But that means taking of those nasty exhaust nuts that snap those crap exhaust studs. Engine out means exhaust off. Try to avoid this.