well.. compresion is not related to the cylinder base gasket .. compression is determined by the piston rings and everything above the rings.
certainly do fix the gasket and also check the head and cylinder surfaces for flatness.. if it has a decompression valve in the head check it's sealing .. check it's cable adjustment.
Check head bolt torque with a torque wrench.. about 10 pount feet or so.
The leak is somewhere up there or it's the rings..They could be carbon-glued and stuck into their grooves.. pull the cylinder off and see. Or the cylinder walls could be glazed .. or something else prevents the rings from sealing. Might just be worn..Try squirting some motor oil around the rings and see of compression improves.
the bike probably just needs a thorough tuneup. the exhaust leak at that joint would bother the hell out of me.. i would do whatever t took to fix that.