Chainsaw Repair

I tried the compresion tester on my neighbour's stihl saw and it measured 125psi which is probably accurate since it is quite an old saw.  This lies between the 110 and 150+ psi value that I have read is to be expected for a chain saw.  My saw reads no more then 65psi.  So its compression!!!!

I dont understand why since the cylinder and piston (what little I can see of it through the muffler hole) looks fine with no scratches.  I have always mixed the oil mixture to be between 50:1 and 33:1 (husqvarna oil) and the machine has been very little used?  I remove the crv and it looked fine.  I added fairy liquid aroung and on the crv and no leaks appeared to show.  I think I would have heard the air escaping anyway?

The only thing I can think of was that the chain oil that I accidently put in the fuel tank might have somehow caused this ( i had tried flushing it out, but maybe the residue that was left caused the problem, not sure how??). 

What causes might there be for the loss of compression?  I'm guessing
a) broken piston ring?
b) worn or badly scratched piston and cylinder?
c) leaking gasget?

Anything else??

I now need to find how to get the cylinder off of the 445.  I have not found any guides on line yet, but I will search later this evening.  I have found a 335 guide..how close will that be to the 445 I wonder?  I will try and get the carb off tomorrow.

I will add to this post what I find when I take the head off.

Thanks again for everyone's help.