Penetrating Oil (WD40 will do) - Drip/spray lots every hour or so for a day - leave overnight - re-spray and leave for 10 minutes, tap with wooden/plastic mallett and it should start to free. Alterntative (if you don't need the old exhaust pipe and sleeves / clamps): Cut off the pipe about 2 inches from the cylinder - cut longways internally (need a decent free-blade metal saw) and, once through, crush with a vice / mole grips and extract. Warning: If you do this, remove cyliner head and barrell and througally clean out any swarf. Rebuild, flush oil through before filling crankcase. Paitence is needed! Good luck.