do a cylinder leak test to determine where
you are losing compression before taking it apart.
a compression check will tell you if one or more is down but not where you are losing it.
turn the engine until the faulty cylinder is at tdc, piston at the top and ready to spark. VERY important that the cylinder is in position ready to spark as the piston comes up twice per cycle. on the wrong cycle the valves will be on overlap and partially open.
now insert into the spark plug hole the home made tool like i made which consists of the bottom half of a spark plug (just the threaded part and nut section) with an air-line fitting welded onto it.
so you now have the ability to put your own compression into the engine with it static.
make sure first though to either lock the flywheel if possible or get someone to put it in gear with the brakes hard on, this will stop the pistons going back down if you haven't quite got it 100% at tdc when the next stage comes...
connect the air-line up and see where the air comes out.
if it's coming out through the exhaust then the exhaust valves are the problem.
out through the carb and it's the inlet valves.
take out the oil filler cap and if it's through there then suspect the pistons/rings/bores.
also check the water expansion tank as if it's bubbling in there then the head gasket/cracked head could be the culprit.
also don't forget to check the plug hole next door as it could be simply blowing between the 2 cylinders.
if you do these checks to determine where you are losing the compression then you will have a clear idea wherer to look when taking it apart
would very much reccomend checking all the items described above, could well be the cause of poor running.